UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


WALNUT  CULTURE  IN  CALIFORNIA 


BY 

L.  D.  BATCHELOR 


BULLETIN  No.  379 

June,  1924 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1924 


WALNUT  CULTURE  IN  CALIFORNIA1 

By  L.  D.  BATCHELOR 


CONTENTS 

PAGE 

General  Review  of  the  Walnut  Industry 4 

Business  Prospects  of  the  Industry 5 

Length  of  life  of  walnut  trees 5 

Profitable  bearing  of  young  trees 5 

Climatic  Requirements 7 

Frost  injury 7 

Heat  injury 8 

Soil  Requirements 9 

Depth  and  character  of  soil 9 

Drainage  and  alkali  injury 10 

Water  Supply........ 12 

Amount  of  irrigation  water  necessary 12 

Irrigation  water  of  good  quality  essential 14 

Varieties 15 

Description  of  varieties 20 

Placentia .• 20 

Eureka 21 

Ehrhardt 24 

Payne 26 

Concord 28 

Franquette 30 

Grove 30 

Rootstocks  and  Choice  of  Nursery  Trees 31 

Choice  of  rootstock 31 

Method  of  propagation 32 

Requirements  for  good  nursery  trees 36 

Starting  the  Young  Orchard ' 36 

Planning  the  arrangement  of  the  orchard 36 

Care  of  trees  before  planting 38 

Planting  the  young  orchard  (nursery  trees) 39 

Planting  the  young  orchard  to  black  walnut  trees  to  be  top-grafted  40 

Training  young  trees 42 

Values  of  orchards  and  lands 48 

Culture 50 

Soil  management 50 

Cover  cropping 53 

Irrigation 58 

Intercropping 68 

Fertilization 73 

Pruning 74 

Insect  and  Disease  Pests 75 

Codling  moth  and  Aphis 75 

Red  Spider 77 

Walnut  blight 78 

Melaxuma 78 

Winter  injury  or  Die-back 82 

Harvesting,  Curing,  Packing,  and  Cost  of  Production 84 

Harvesting 84 

Washing  and  curing 85 

Advantages  of  dehydration 87 

Packing 88 

The  standard  for  first-grade  nuts 89 

Selling  the  crop 89 

Income  to  be  derived  from  California  walnuts 91 

1  Paper  No.  118,  University  of  California,  Graduate  School  of  Tropical  Agri- 
culture and  Citrus  Experiment  Station,  Riverside,  California. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

PREFACE 

This  bulletin  represents  the  results  of  general  observations  and 
specific  investigations  of  the  writer  and  his  associates.  The  subject 
matter  and  the  method  of  presentation  have  been  chosen  in  response 
to  the  numerous  inquiries  relating  to  general  walnut  culture,  which 
have  been  received  during  the  past  nine  years. 

GENERAL  REVIEW  OF  THE  WALNUT  INDUSTRY 

The  Persian  walnut  (Juglans  regia)  or,  as  it  is  more  commonly 
known,  the  English  walnut,  may  be  seen  growing  in  nearly  every 
county  in  the  state  of  California.  The  commercial  production  of  nuts, 
however,  is  centered  mainly  in  southern  California  in  the  following 
four  counties,  mentioned  in  the  order  of  importance:  Los  Angeles, 
Orange,  Ventura,  and  Santa  Barbara.  Riverside  and  San  Bernardino 
counties  have  a  considerable  area  of  young  groves,  and  in  the  central 
part  of  the  state  walnut  culture  is  gradually  becoming  important  in 
Santa  Clara,  San  Joaquin,  Contra  Costa,  Napa,  and  Sonoma  counties. 

At  the  present  time,  there  are  approximately  100,000  acres  of 
walnut  trees  of  various  ages  within  the  state.  The  annual  production 
for  the  state  during  the  last  15  years  is  shown  by  table  1. 


TABLE  1 

Walnut  Production  and  Prices  for  Fifteen  Years 
(Prices  F.O.B.  California) 


Year 

Production 
(Pounds) 

Price  per  lb. 

No.  1  Soft  Shells 

(Cents) 

Price  per  lb. 

Fancy  Budded 

(Cents) 

1909 

18,700,000 
19,200,000 
25,000,000 
22,500,000 
22,700,000 
17,800,000 
29,650,000 
29,200,000 
33,000,000 
40,230,000 
56,200,000 
43,000,000 
35,916,000 
48,000,000 
48,930,000 

11.5 
15.0 
14.0 
14.0 
16.0 
16.5 
13.6 
15.5 
20.1 
28.0 
31.5 
22.5 
24.5 
22.5 
22.0 

15  0 

1910 

17.5 

1911 

16.5 

1912 

16.5 

1913 

19.0 

1914 

20.0 

1915 

17.0 

1916 

19.0 

1917 

24.1 

1918 

31.5 

1919 

34.0 

1920 

25.5       ' 

1921 

28.0 

1922 

26.0 

1923 

26.0 

Average 

19.53 

22.37 

Bulletin  379]  WALNUT  CULTURE  IN   CALIFORNIA  5 

BUSINESS  PROSPECTS  OF  THE   INDUSTRY 

The  present  condition  of  the  industry  indicates  that  walnuts  may 
continue  to  be  one  of  the  most  staple  and  at  the  same  time  the  most 
profitable  crops  which  can  be  grown  on  land  and  in  locations  adapted 
to  this  crop.  The  annual  consumption  of  walnuts  in  the  United  States 
has  ranged  between  60,000,000  and  95,000,000  pounds  annually  during 
the  past  few  years,  and  has  increased  100  per  cent  since  1909.  Nuts 
which  were  formerly  looked  upon  as  a  holiday  luxury  are  becoming 
more  and  more  a  food  to  be  used  the  year  through  in  the  average  house- 
hold. This  increase  and  wider  use  of  the  walnut  has  been  accompanied 
by  an  increase  in  importation  exceeding  our  own  average  production, 
and  a  gradual  increase  in  the  prices  paid  to  the  California  grower. 

LENGTH  OF  LIFE  OF  WALNUT  TREES 

The  limit  of  the  profitable  length  of  life  of  a  walnut  grove  cannot 
be  determined  from  experiences  in  California.  Provided  the  soil, 
climatic  and  water  conditions  are  well  suited  to  this  crop,  the  trees 
may  continue  to  thrive  and  produce  satisfactory  crops  for  a  long 
period.  Groves  which  are  now  35  to  40  years  old  are  still  among  the 
most  prolific  in  the  state.  Notable  examples  of  such  old,  yet  produc- 
tive groves  are  the  J.  C.  Daly  grove  of  20  acres  planted  in  1881,  18 
acres  of  the  J.  M.  Sharp  grove  planted  in  1887,  and  the  T.  A.  Kelsey 
grove  of  26  acres  planted  in  1888.  The  last  mentioned  grove  has  only 
12  trees  to  the  acre,  planted  60  feet  apart  each  way ;  the  trees  at  their 
present  age  of  36  years  show  no  apparent  deterioration.  Groves  in 
which  the  trees  are  planted  60  feet  each  way  may  be  expected  to  grow 
longer  without  deterioration  than  groves  where  the  trees  are  planted 
closer  and  thus  severely  compete  with  each  other. 

The  writings  of  travelers  in  Asia  and  Europe  frequently  describe 
walnut  trees  from  one  to  two  hundred  years  old.  The  continued  health 
and  profitableness  of  many  of  the  present  day  California  walnut  groves 
until  they  are  a  century  old  seems  a  reasonable  expectation.  Much 
depends,  as  will  be  related  later,  upon  the  soil,  drainage,  and  general 
climatic  conditions. 

PROFITABLE   BEARING  OF   YOUNG  TREES 

The  young  walnut  grove  may  be  expected  to  bear  profitable  crops 
by  the  time  the  trees  are  6  to  10  years  old,  according  to  the  variety,  the 
number  of  trees  planted  per  acre,  and  the  natural  conditions  surround- 
ing the  grove.  The  following  cases  may  illustrate  the  possibilities  of 
early  production  in  young  walnut  groves. 


6  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

A  grove  of  100  acres  of  the  Placentia  variety  in  Orange  County, 
produced  an  average  of  44  pounds  of  ungraded  nuts  per  tree  in  its 
ninth  year.  As  the  trees  are  60  feet  apart  this  represents  528  pounds 
per  acre.  During  the  first  10  or  12  years  this  might  well  have  been 
double  planted  and  thus  produced  approximately  1056  pounds  pe 
acre  the  ninth  year. 

A  block  of  60  trees  of  the  Chase  variety  on  a  heavy  clay  loam  soil 
in  the  Puente  district,  produced  a  profitable  crop  of  35  pounds  per 
tree  in  the  sixth  year.  The  records  of  these  groves  and  of  a  grove  of 
38  Ehrhardt  trees  are  shown  in  table  2. 

In  considering  this  table  one  should  keep  in  mind  that  each  of 
these  plantations  is  growing  under  the  best  natural  and  cultural  con- 
ditions, and  is  thus,  without  doubt,  considerably  above  the  average. 
In  projecting  a  business  enterprise,  the  yields  shown  may  be  looked 
upon  as  approaching  the  maximum  possibilities,  and  should,  therefore 
be  discounted  25  to  50  per  cent  to  reach  a  figure  which  may  reasonably 
be  expected  to  prevail  under  average  conditions.  Again,  the  figures 
here  presented  are  of  little  value  in  making  comparisons  between  the 
three  varieties  in  question,  as  no  two  varieties  were  of  the  same  age 
the  same  season. 

TABLE   2 
Average  Yearly  Production  Per  Tree  of  Young  Walnut  Groves 


Variety 

Placentia 

Chase 

Ehrhardt 

Year 

Season's 
growth 

Yield 
in  lbs. 

(Ungraded  Nuts  at  harvest) 

(Ungraded  Nuts  at  harvest) 

Season's 
growth 

Yield 
in  lbs. 

Season's 
growth 

Yield 
in  lbs. 

1915 

7th 

8th 

9th 

10th 

11th 

12th 

22f 

27f 

44* 

43* 

104* 

82* 

10th 
11th 
12th 
13th 
14th 
15th 

80 

1916 

76 

1917 

5th 
6th 
7th 
8th 

18 
35 

72 
43 

79 

1918.... 

81 

1919... 

117 

1920... 

98 

*  Pounds  of  uncured  and  ungraded  nuts  at  harvest, 
t  Pounds  of  cured  and  ungraded  nuts  at  harvest. 


Although  the  cultural  and  natural  conditions  in  all  three  groves 
were  favorable,  the  plantings  are  several  miles  apart  and  conditions 
were  not  identical.  Certain  seasons  are  particularly  favorable  to  crop 
production.    The  year  1919  was  characterized  by  especially  abundant 


Bulletin    379]  WALNUT    CULTURE    IN    CALIFORNIA  7 

crops  of  walnuts.  These  were  followed  in  many  cases  by  lighter  crops 
in  1920,  although  the  young  trees  were  one  year  older  and  had  a 
greater  bearing  surface. 

CLIMATIC  REQUIREMENTS 

The  chief  climatic  limitations  of  the  walnut  are  frosts  in  spring 
and  fall,  and  extreme  heat  in  summer. 

FROST  INJURY 

Low  frosty  sites  should  be  avoided  for  walnuts.  A  temperature  of 
even  2  or  3  degrees  below  freezing  (32°  F.)  for  a  period  of  an  hour  or 
more  will  usually  kill  a  large  percentage  of  the  young  walnut  flowers, 
if  they  are  just  starting  to  bloom.  During  the  season  of  1920,  late 
spring  frosts  during  the  second  week  of  April  reduced  the  walnut 
crop  to  a  great  extent  in  several  of  the  inland  valleys.  In  one  instance 
under  careful  observation  the  absolute  minimum  recorded  by  a  reliable 
thermometer,  was  only  29°  F.,  yet  it  was  estimated  that  50  per  cent  of 
the  crop  was  destroyed.  During  this  period  apricots  and  the  earlier 
blooming  cling  peaches  in  adjacent  orchards,  were  about  the  size  of 
green  peas,  and  experienced  little  or  no  injury. 

During  the  morning  of  April  6,  1921,  a  minimum  temperature  of 
27°  F.  was  experienced  by  a  seedling  walnut  grove  in  which  the. 
majority  of  the  trees  were  in  full  bloom.  This  temperature  prevailed 
for  about  an  hour  and  apparently  destroyed  nearly  the  whole  crop, 
except  in  the  tops  and  centers  of  the  trees,  where  an  occasional  cluster 
of  blooms  escaped  injury. 

As  a  general  rule,  the  late  blooming  varieties,  such  as  the  Eureka 
and  Grove,  are  less  liable  to  injury  from  the  late  spring  frosts  than 
the  earlier  blooming  soft-shell  types.  During  the  spring  of  1915,  how- 
ever, walnut  twig  tips  and  young  nuts  were  killed  during  the  first 
week  in  May  on  some  of  the  low  sites  in  Los  Angeles  County.  With 
a  frost  as  late  as  May  the  late  blooming  varieties  may  suffer  more 
injury  than  the  earlier  ones,  as  the  young  nuts  (one-half  inch  or  more 
in  diameter)  of  the  early  blooming  varieties,  at  this  time  will  stand 
more  cold  than  the  blossoms  of  the  late  varieties.  The  late  blooming 
varieties  do  not,  therefore,  always  escape  late  spring  frosts.  It  is 
better  to  avoid  planting  walnuts  on  land  which  is  considered  too 
frosty  for  peaches.  The  walnut  varieties  of  the  soft-shell  type  bloom 
about  the  same  time  as  the  Bartlett  pear  but  their  flowers  are  not 
nearly  so  resistant  to  frost  injury. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Early  fall  frosts  also  cause  injury,  especially  to  young  shoot  growth, 
although  the  injury  may  not  be  apparent  to  the  casual  observer  until 
the  following  spring,  when  the  tree  will  fail  to  leaf  out  on  the  injured 
portion  of  the  past  season 's  growth.  This  type  of  injury  is  more  fully 
treated  under  the  subject  of  die-back.  It  may  suffice  to  explain  here 
that  early  fall  frosts  prematurely  stop  the  final  ripening  process  of 
the  twigs  by  killing  the  leaves.  Such  injured  leaves  will  usually  fall 
off  within  a  week  after  the  frost  injury,  whereas  they  might  otherwise 
have  functioned  normally  a  month  or  six  weeks  longer.  The  bare, 
immature  twigs  dry  out  rapidly  and  sunburn  on  the  south  side  during 
the  winter  months,  dying  back  several  inches,  or  possibly  several  feet 
to  the  more  mature  wood,  which  has  been  able  to  withstand  the 
abnormal  exposure. 

HEAT  INJURY 

The  chief  injury  to  the  walnuts  during  the  extreme  hot  weather  of 
the  summer  months  is  the  sunburning  of  the  nuts  located  on  the  out- 
sides,  tops,  and  south  sides  of  the  trees.  Such  burned  nuts  usually 
become  "blanks"  if  the  injury  occurs  during  June  or  July.  If  the 
sunburning  takes  place  when  the  nuts  are  more  fully  developed,  in 
August,  they  may  be  salvaged  as  culls  with  a  portion  of  the  kernels 
edible.  In  mild  cases  of  sunburn  injury  a  small  part  of  the  husk  may 
stick  to  the  shell  or  may  only  stain  it,  causing  the  nut  to  be  graded 
out  as  a  cull.  The  kernels  of  such  nuts  may  be  of  good  quality,  although 
it  is  likely  that  there  will  occur  a  high  percentage  of  amber  and  black 
kernels,  and  partly  shriveled  and  mouldy  meats  among  this  class  of 
nuts.  Nuts  on  trees  growing  on  a  deep  loam  soil  withstand  the  extreme 
heat,  without  burning,  much  better  than  nuts  on  trees  growing  on  a 
shallow  or  sandy  soil  with  less  moisture  available.  Sandy  soil  must 
be  frequently  irrigated  in  the  hot  sections  if  the  walnuts  are  to  avoid 
serious  injury  by  sunburning.  A  temperature  of  100°  F.,  or  above, 
accompanied  by  low  humidity  will  usually  cause  walnuts  to  sunburn 
if  they  are  exposed  to  the  direct  rays  of  the  sun.  This  temperature,  or 
higher,  occurs  in  many  of  the  inland  valleys  15  to  30  days  during 
the  summer  months.  Under  such  conditions  the  crop  is  always  injured 
more  or  less  by  sunburn.  Regardless  of  this,  the  planting  of  walnuts 
is  extending  more  and  more  inland.  The  planting  of  Valencia  oranges 
is  restraining  the  development  of  walnut  groves  in  the  coastal  sections. 
At  the  present  time  walnut  groves  have  not  become  so  productive  in 
the  inland  valleys,  which  experience  frequent  summer  days  with  maxi- 
mum temperatures  of  100°  F.  and  over,  as  they  have  been  nearer  the 
coast,  where  the  weather  is  characterized  by  a  smaller  daily  range  and 


Bulletin    379]  WALNUT    CULTURE    IN    CALIFORNIA  9 

a  lower  maximum  temperature.  Varieties  especially  selected  for  the 
hotter  sections  may  tend  to  correct  this  difficulty  in  the  future.  In 
the  past,  most  of  the  walnuts  planted  inland  have  been  of  the  same 
varieties  as  those  which  were  originally  selected  for  the  coastal  regions. 
Varietal  adaptation  is  more  fully  treated  under  the  heading  of  varieties. 

At  present,  walnut  culture  is  most  successful  in  the  sections  of  the 
state  which  are  characterized  by  a  long  growing  season  free  from  frosts, 
where  the  daily  range  in  temperature  is  not  great,  where  the  absolute 
maximum  seldom  goes  above  100°  F.,  and  where  winter  temperatures 
are  moderate  and  the  humidity  is  high,  especially  during  the  summer. 
Sudden  changes  in  temperature,  such  as  occur  in  the  desert  regions, 
seem  to  be  decidedly  detrimental  to  the  growth  of  the  walnut  tree. 
The  localities  where  favorable  climatic  conditions  occur  are  indicated 
by  the  largest  walnut  shipping  centers  in  the  state  which  are  Santa 
Ana,  Whittier,  Puente,  Saticoy,  and  Santa  Barbara. 

When  the  tree  is  fully  dormant,  the  absolute  minimum  temperatures 
of  winter,  during  January  and  February,  apparently  have  very  little 
to  do  with'  its  welfare.  This  is  especially  true  of  the  French  varieties. 
Trees  of  the  Mayette  and  Franquette  varieties  have  grown  for  some 
years  without  severe  winter  injury  along  the  west  side  of  the  Wasatch 
Mountains  north  of  Salt  Lake  City,  Utah,  where  the  minimum  tem- 
perature occasionally  reaches  —  5°  F. 


SOIL  REQUIREMENTS 
DEPTH  AND  CHAEACTEE  OF  SOIL 

The  success  of  walnut  culture  is  dependent  upon  favorable  soil 
conditions  to  a  greater  extent  than  that  of  many  other  tree  crops, 
A  well-drained  deep  silt  loam,  containing  an  abundance  of  organic 
matter,  free  from  a  high  or  fluctuating  water  table,  hardpan,  sandy 
subsoil,  and  ' '  alkali, ' '  is  essential  for  a  first-class  walnut  grove.  Shal- 
low soils  underlaid  by  a  hardpan,  sand  layer,  or  an  impervious  clay 
stratum  w7ithin  5  or  6  feet  from  the  surface  will  produce  only  a  short- 
lived, second-rate,  walnut  grove.  Such  groves  are  frequently  stunted 
in  growth,  subject  to  yellowness,  and  are  only  fair  producers. 

There  are  a  few  moderately  successful  groves  growing  in  fine  silt 
soil  underlaid  with  fine  sand  within  4  or  5  feet  from  the  surface,  but 
such  properties  require  the  greatest  skill  in  planning  the  cultural  and 
irrigation  practice.  A  walnut  grove  growing  on  such  soil  has  a  con- 
siderably lower  intrinsic  value  than  one  growing  under  good  soil 
conditions  if  the  other  environmental  factors  are  equal. 


10  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Very  light  sandy  soils  are  usually  unsatisfactory,  producing  slow- 
growing,  stunted,  poor-yielding  trees.  On  such  soils  the  nuts  are  very 
subject  to  sunburning.  One  should  not  judge  the  soil  conditionr 
entirely  by  the  surface  soil.  The  nature  of  the  subsoil  may  be  of 
great  importance.  There  are  first-class  walnut  groves  in  the  El  Montr 
district  growing,  apparently,  in  a  very  light  sandy  soil.  The  soil 
changes,  however,  in  some  instances  3  or  4  feet  from  the  surface,  grad- 
ing into  a  silt  loam  which  seems  to  be  an  ideal  soil  for  walnut  roots. 

Very  heavy  adobe  soil  or  clay  loam  underlaid  by  adobe  are  not  well 
suited  to  walnut  groves.  The  trees  on  such  soils  are  usually  somewhat 
stunted,  and  make  very  little  growth  of  fruit  wood  in  the  tops  when 
mature.  Heavy  soils  are  difficult  to  cultivate,  and  it  is  hard  to  obtain  a 
deep  penetration  of  the  irrigation  water  which  is  so  essential  to  good 
walnut  culture.  Such  soils  frequently  contain  amounts  of  alkali  salts 
which  are  harmful  to  walnuts,  and  their  leaching  by  any  system  of 
irrigation  is  very  difficult. 

DRAINAGE  AND  ALKALI  INJURY 

A  water  table  within  9  or  10  feet  from  the  surface  of  the  soil  may 
make  it  impossible  to  maintain  a  profitable  walnut  grove.  Most  of 
the  high  water  tables  in  arid  regions  are  characterized  by  carrying 
more  or  less  * ' alkali"*  in  solution.  The  mere  presence  of  water  within 
9  or  10  feet  of  the  surface  is  usually  only  part  of  the  difficulty,  the 
alkali  present  being  of  greater  concern,  for  the  deep-rooted  trees  obtain 
a  portion  of  their  water  from  this  water  table,  and  may  be  injured  by 
the  salts  therein,  even  though  there  is  a  seemingly  sufficient  layer  of 
good  soil  above  the  water.  An  instance  occurred  in  Orange  County 
where  an  old  grove  was  practically  ruined  by  a  rise  in  the  water  table 
to  within  only  nine  feet  of  the  surface.  This  water  contains  700  parts 
per  million  of  salts.  The  water  rises  approximately  6  feet  by  capillary 
action  in  the  soil ;  thus  there  is  only  a  3-foot  depth,  or  one-third  of  the 
soil,  above  the  standing  water,  which  is  not  impregnated  with  salts. 
Several  instances  have  also  been  noted  in  Ventura  County  where  wal- 
nut trees  were  severely  injured  by  the  presence  of  an  ' '  alkali ' '  ground 
water  9  to  13  feet  from  the  surface.  When  the  trees  were  planted  20 
years  ago  in  this  district,  the  water  was  at  least  30  feet  from  the 
surface.  It  is  difficult  to  place  a  definite  safe  boundary  beyond  which 
injury  from  a  high  water  table  is  likely  to  occur. 


*  For  a  discussion  of  alkali  in  the  irrigation  water  see  page  14. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


11 


Apparently,  quite  as  much  depends  upon  the  nature  of  the  water 
as  upon  the  actual  depth  from  the  surface,  within  the  root  zone.  The 
rainfall,  irrigation  practice,  the  nature  of  the  soil  and  of  the  irriga- 
tion water  may  also  greatly  influence  the  suitability  of  land  with  a 
high  water  table  for  walnut  culture. 

Several  examples  of  groves  severely  injured  by  an  alkali  water 
table  have  been  observed  to  improve  to  a  marked  degree  after  install- 
ing tile  drainage.  Drains  should  be  laid  6  to  7  feet  deep  to  effectively 
improve  conditions.  After  the  installation  of  tile  drains,  relatively 
frequent  and  moderate  amounts  of  irrigation  water  should  be  applied 


Fig.  1. — Effect  of  application  of  alkali  water.     Grove  on  left  used  water  of  high 
salt  content,  that  on  right,  good  water. 

so  that  large  amounts  of  the  alkali  water  from  the  water  table  will  not 
be  drawn  upward  by  capillary  action  above  the  tile  system.  By  fol- 
lowing such  an  irrigation  practice  the  salt  concentration  in  the  upper 
six  feet  of  soil  will  be  reduced  to  a  point  which  the  walnuts  will 
tolerate. 

The  groves  which  have  been  tile  drained  and  judiciously  irrigated 
have  not  returned  to  a  normal  condition,  however.  Some  alkali  injury* 
is  noticeable  among  them,  and  although  the  crop  production  has 
returned  to  a  profitable  commercial  tonnage,  it  seems  unlikely  that  a 
first-class  grove  can  be  maintained  under  such  conditions. 

Groves  being  somewhat  injured  by  poor  drainage  conditions,  even 
though  tile  drained,  have  shown  a  marked  improvement  after  being 


*  For  a  more  detailed  discussion  of  the  characteristics  of  alkali  injury  see 
page  14. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


intercropped  with  alfalfa*  for  several  years.  The  cause  of  the  bene- 
ficial effect  of  such  a  crop  association  is  not  fully  understood,  but  the 
improvement  following  such  plantings  has  been  apparent  in  so  many 
instances  that  the  results  can  not  be  explained  by  mere  chance.  Alfalfa 
will  thrive  on  alkali  soil  conditions  far  beyond  the  tolerance  of  walnut 
trees.  The  deep-rooted  alfalfa  plants,  no  doubt,  use  up  a  portion  of  the 
alkali  salts  present.  The  presence  of  the  alfalfa  makes  the  flooding  of 
the  land  by  irrigation  water  more  practicable,  and  probably  results 
in  a  more  uniform  downward  movement  of  soil  moisture  with  less 
liability  of  salt  concentration  in  localized  areas,  a  condition  which  is 
often  typical  of  furrow  irrigation.  The  shading  of  the  soil  surface  by 
the  alfalfa,  reduces  the  surface  evaporation  and  thus  hinders  the  rise 
of  capillary  water  from  the  water  table  below.  It  is  impossible,  how- 
ever, to  state  which  of  the  above  mentioned  factors  are  most  potent 
in  causing  the  beneficial  effect. 

Exceptions  to  the  rule  of  injury  resulting  from  a  high  water  table 
are  very  rarely  found.  There  are  very  occasional  examples  where 
orchards  are  subirrigated  by  a  natural  flow  of  water  from  the  hills,  or 
natural  stream  beds.  Judging  from  the  appearance  of  the  trees  in 
several  of  these  rare  examples  this  ground  water  contains  no  (or  an 
exceedingly  small  amount  of)  "alkali,"  a  condition  which  is  excep- 
tional  in  the  usual  high  water  tables  in  irrigated  areas  of  the  arid  west. 

WATER   SUPPLY 

In  most  of- the  California  walnut  districts,  irrigation  is  necessary. 
"The  amount  of  water  to  be  applied  and  the  seasons  of  application 
naturally  depend  upon  the  rainfall,  the  amount  of  evaporation,  the 
nature  of  the  soil,  and  other  factors.  The  walnut  tree  can  not  be 
expected  to  yield  profitable  crops  unless  furnished  with  an  abundant 
supply  of  water  during  practically  the  entire  year. 


•    TABLE  3 

Showing  the  Normal  Kainfall  in  Several  Walnut  Growing  Sections 


Place  of  Observation 

Normal  Annual 

Rainfall  in 

Inches 

Place  of  Observation 

Normal  Annual 

Rainfall  in 

Inches 

Los  Angeles 

14.6 
24.3 
19.2 
10.6 
17.1 

San  Diego : 

10.0 

Napa  ...        

San  Jose 

16,8 

Pomonaf      

Santa  Ana  (Irvine) 

13.3 

Riverside 

Santa  Rosa. .-...,. ......'. 

32.0 

Santa  Barbara 

Stockton r. 

14.6- 

*  For  a  discussion  of  injurious  effects  of  alfalfa  on  normal  groves  see  page  71. 
t  Average  of  eight  years,  1913  to  1920,  inclusive. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


13 


AMOUNT  OF  IRRIGATION  WATER  NECESSARY 

From  12  to  24  acre-inches  per  acre*  of  irrigation  water  is  usually 
applied  to  bearing  walnut  groves,  the  amount  varying  according  to 
some  of  the  conditions  related  in  the  preceding  and  the  following 
paragraphs. 

The  amount  of  water  necessary  for  good  walnut  culture  will  vary 
with  the  nature  of  the  soil,  as  an  infertile  soil  requires  more  water  to 
produce  equivalent  crops  than  a  rich  soil.  Closely  planted  and  old 
bearing  groves  will  require  more  water  than  young  groves,  or  those 
with  fewer  trees  to  the  acre,  other  things  being  equal. 

Table  3  shows  the  average  rainfall  in  several  of  the  sections  where 
walnuts  are  grown. 

While  the  amount  of  seasonal  rainfall  may  be  taken  as  a  good 
indication  of  the  amount  of  irrigation  water  necessary  to  supplement 
it,  it  should  be  borne  in  mind  that  other  factors,  such  as  frequency, 
duration,  amount  of  individual  rains,  run-off,  and  season  of  occurrence 
must  also  be  considered,  as  they  influence  the  proportion  of  the  total 
precipitation  which  penetrates  into  the  soil  and  is  available  to  the 
walnut  trees. 


*  For  a  detailed  description  of  the  various  methods  of  measuring  water  of 
the  farm,  see  University  of  California  College  of  Agriculture,  &_gricultural 
Experiment  Station  Circular  250'.  X 

An  acre-inch  of  water  is  the  quantity  of  water  required  to  cover  one  acre 
to  a  depth  of  one  inch.  i  . 

50  California  lt miners'  inches  "==1  cubic  foot  per  second. 

1  cubic  foot  per  second=7.48  gallons  per  second. 

Miners'  Inches  to  Acre  Inches 


Hours 

Miners'  Inches 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

1 

.0198 
.0396 
.0594 
.0792 
.0990 
.1188 
.1386 
.1584 
.1782 
.1980 
.2178 
.2376 

.0396 
.0792 
.1188 
.1584 
.1980 
.2376 
.2772 
.3168 
.3564 
.3960 
.4356 
.4752 

.0594 
.1188 
.1782 
.2376 
.2970 
.3564 
.4158 
.4752 
.5346 
.5940 
.6534 
.7128 

.0792 
.1584 
.2376 
.3168 
.3960 
.4752 
.5544 
.6336 
.7128 
.7920 
.8712 
.9504 

.0990 
.1980 
.2970 
.3960 
.4950 
.5940 
.6930 
.7920 
.8910 
.9900 
1.0890 
1 . 1880 

.1188 

.2376 

.3564 

.4752 

.5940 

.7128 

.8316 

.9504 

1.0692 

1 . 1880 

1 . 3068 

1.4256 

.1386 

.2772 

.4158 

.5544 

.6930 

.8316 

.9702 

1 . 1088 

1 . 2474 

1 . 3860 

1 . 5246 

1 . 6632 

.1584 

.3168 

.4752 

.6336 

.7920 

.9504 

1 . 1088 

1.2672 

1.4256 

1.5840 

1.7424 

1.9008 

.1782 

.3564 

.5346 

.7128 

.8910 

1.0692 

1 . 2474 

1.4256 

1.6038 

1 . 7820 

1 . 9602 

2.1384 

.1980 

2 

.3960 

3 

.5940 

4 

. 7920      S 

5 

.9900     "o 

6 

1 . 1880     ~ 

7 

1 . 3860     § 

8 :.. 

1 . 5840     < 

9 

1 . 7820 . 

10 

1 . 9800 

11 

2.1780 

12 

2.3760 

14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Monthly  records  of  the  average  penetration  of  rain  water  into  the 
soil  were  kept  for  three  years  in  Riverside  County,  in  a  walnut  grove 
growing  on  a  sandy  loam  soil.  The  results  varied  somewhat  from 
month  to  month,  according  to  frequency,  duration,  and  volume  of 
rainfall.  The  total  seasonal  penetration  into  the  soil  varied  from 
4  to  6  inches  with  an  average  of  5.2  for  each  inch  of  rain.  The  efficacy 
of  many  of  the  light  rain  storms  of  one-half  an  inch  in  volume  and 
less,  is  greatly  reduced  by  the  evaporation  from  the  surface  soil.  The 
penetration  of  soil  moisture  per  inch  of  rainfall  will  be  considerably 
greater  than  the  above  records  show  during  seasons  when  the  storms 
occur  with  short  intervals  between  them,  or  when  the  individual  storms 
are  of  uncommonly  large  volume.  The  depth  of  penetration  of  winter 
rains  will  also  depend  to  some  degree  on  the  initial  dryness  of  the 
soil  as  well  as  on  its  water  holding  capacity.  For  a  discussion  of  this 
latter  side  of  the  problem,  see  the  subject  of  water  holding  capacity  of 
soils,  page  65. 

The  water  requirements  are  higher  in  the  warm  inland  valleys  with 
their  very  low  humidity  and  high  temperatures  during  the  growing 
season,  than  in  the  coastal  regions,  with  their  cooler  summers  and 
many  foggy  days. 

During  years  of  subnormal  rainfall  the  water  requirements  noted 
above  will  prove  inadequate  and  provision  should  be  made  for  heavy 
application  of  water  amounting  to  as  much  as  12  acre-inches  per  acre 
in  very  dry  years  during  the  winter  months.  (See  winter  irrigation, 
page  82.) 

It  may  be  safely  said,  therefore,  that  the  walnut  grower  in  some 
of  the  southern  and  central  inland  districts  should  be  provided  with  a 
water  right  of  24  acre-inches  per  acre  per  year,  during  years  of  normal 
rainfall,  and  that  from  this  maximum  the  requirements  diminish  until 
in  some  individual  and  exceptional  cases  in  Sonoma  and  Napa  counties, 
walnuts  may  be  grown  successfully  without  any  irrigation. 

IRRIGATION  WATER  OF  GOOD  QUALITY  ESSENTIAL 

The  quality  of  the  irrigation  water  for  walnuts  is  as  important  as 
the  quantity.  Relatively  small  amounts  of  the  "alkali"  salts  carried 
in  the  water  are  decidedly  harmful  to  walnut  trees.  From  many  obser- 
vations made  during  the  past  eight  years,  it  seems  that  the  walnut 
tree  is  the  most  sensitive  to  alkali  injury  of  all  the  orchard  trees  grown 
in  California.  The  minimum  amount  of  salts  which  will  prove  toxic 
to  walnut  trees  will  depend  upon  so  many  factors  such  as  the  nature 
of  the  soil,  the  under-drainage,  the  rainfall,  etc.,  that  it  may  be  impos- 


Bulletin  379]  walnut  culture  in  California  15 

sible  to  state  the  exact  maximum  amount  of  salt  which  might  be  applied 
with  the  water  without  injury  to  the  trees.  In  the  districts  of  heavy 
rainfall  and  in  well  underdrained  groves,  water  of  poor  quality  could 
no  doubt  be  used  longer  without  harm  to  the  trees  than  in  the  more  arid 
regions,  especially  where  hardpan  exists.  It  is  definitely  known  that 
walnut  groves  have  been  injured  severely  by  the  use  of  irrigation  water 
containing  only  340  parts  per  million  of  one  or  more  of  the  so-called 
"alkali"  salts.* 

The  particular  salts  which  the  water  contains  may  have  as  great 
a  bearing  on  the  injury  as  the  type  of  soil  and  other  natural  conditions 
of  the  orchard.  It  is  impracticable  to  include  a  prolonged  discussion 
of  the  effect  of  alkali  on  walnuts  in  this  publication  ;  suffice  it  to  warn 
the  present  or  prospective  walnut  grower  of  the  importance  of  a  water 
supply  of  good  quality  before  continuing  or  establishing  a  walnut 
plantation.  It  is  especially  important  in  projecting  new  plantings  to 
have  the  prospective  water  supply  analyzed f  and  an  expert  opinion 
given  upon  its  suitability,  before  proceeding  to  develop  the  property. 

Groves  which  have  been  severely  injured  by  irrigation  water  con- 
taining harmful  amounts  of  alkali  salt  may  be  expected  to  recover  to 
a  large  degree,  in  fact  possibly  to  a  normal  condition  after  several 
years  use  of  good  water.  The  length  of  time  necessary  for  such  recov- 
ery will  depend  on  several  factors  aside  from  the  extent  of  the  original 
injury,  such  as  nature  of  the  soil,  subsoil,  and  drainage  conditions, 
amount  of  water  used,  and  method  of  irrigation  followed  after  chang- 
ing to  a  good  water  supply.  Several  groves  have  been  observed  which 
were  so  injured  that  the  production  had  reached  nearly  zero.  After 
three  years'  use  of  good  water,  combined  with  good  culture,  recovery 
has  reached  a  point  of  profitable  bearing.  Figure  1  shows  the  effect 
of  using  alkali  water  for  irrigating  walnut  trees. 


VARIETIES 

The  original  walnut  plantings  in  California  were  the  ' '  hardshell ' ' 
type  of  seedling  trees.  This  type  was  grown  as  dooryard  and  roadside 
trees,  before  walnuts  were  planted  in  commercial  orchards. 


*  For  a  discussion  of  alkali  salts  in  irrigation  water,  see  Kelley  and  Thomas, 
"The  Effects  of  Alkali  on  Citrus  Trees,"  Univ.  of  Calif.  Experiment  Station 
Bull.  No.  318,  1920'. 

t  The  University,  through  its  division  of  Plant  Nutrition  at  Berkeley  or 
the  chemical  division  of  the  Citrus  Experiment  Station  at  Biverside,  is  prepared 
to  analyze  irrigation  waters  free  of  charge,  and  give  an  opinion  on  their  suita- 
bility for  irrigation  purposes. 


16  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  history  of  the  commercial  walnut  culture  in  California  com- 
menced in  1867  when  the  late  Joseph  Sexton  of  Santa  Barbara  planted 
part  of  a  sack  of  walnuts.  These  walnuts  were  purchased  in  San 
Francisco  and  were  probably  imported  from  Chile.  From  this  original 
planting  came  hard-shell  and  paper-shell  types  of  trees.  Among  the 
second  generation  trees  from  nuts  from  this  planting  came  the  "Santa 
Barbara  Soft-Shell"  type  of  tree  which  bore  nuts  in  many  respects 
intermediate  between  the  two  types  which  appeared  in  the  original 
planting.  The  tree  characteristics  of  this  new  type  were  soon  recog- 
nized to  be  superior  to  either  of  the  original  types.  The  ' '  soft-shelled ' ' 
type  of  tree  grew  more  vigorously  and  was  more  prolific  than  the  other 
types.  Combined  with  this,  the  nuts  were  somewhat  larger  and  had  a 
more  desirable  thickness  of  shell  than  either  the  "hard-shell"  or 
1 '  paper-shell ' '  types. 

Seedling  trees  were  grown  from  the  best  of  the  Santa  Barbara  soft- 
shell  trees,  and  thus  began  the  early  development  of  the  present  walnut 
industry,  which  was  based  entirely,  in  southern  California,  on  seedling 
trees,  the  ultimate  origin  of  which  traced  back  to  the  Sexton  planting. 
During  the  period  of  rapid  development  of  the  new  industry  less  atten- 
tion was  paid  to  the  parentage  of  the  seedling  nursery  trees  than  had 
been  the  practice  of  the  pioneer  planters.  The  great  demand  for  trees 
made  it  possible  to  sell  trees  of  unknown  parentage,  thus  many  of  the 
seedling  groves  planted  20  to  25  years  after  the  industry  was  well 
started  are  decidedly  inferior  to  the  older  groves  regarding  produc- 
tivity, size  and  vigor  of  trees,  uniformity  and  type  of  nuts.  The  writer 
knows  of  no  seedling  grove  which  was  planted  during  the  decade  of 
1890  to  1900,  or  later,  which  approaches  in  uniformity  of  trees  and 
nuts,  the  oldest  groves  in  Ventura  and  Santa  Barbara  counties  which 
were  planted  10  to  15  years  earlier. 

The  collections  of  heterogeneous  trees,  regarding  both  the  type  of 
nuts  as  well  as  the  bearing  qualities  composing  the  majority  of  the 
seedling  groves  now  in  existence,  finally  forced  the  industry  to  the 
propagation  of  trees  by  grafting.  The  most  observing  growers  and 
nurserymen  selected  particularly  meritorious  seedling  trees  from  which 
to  propagate.  Thus  arose  the  clonal  varieties  of  walnuts  in  southern 
California,  practically  all  of  which  are  (except  the  Eureka,  see  page 
21)  lineal  dependents  of  the  original  Sexton  planting.  Such  varieties 
include  the  Placentia,  Neff,  Prolific,  and  probably  the  Chase.  During 
the  decade  of  1895  to  1905  the  propagation  of  walnuts  gradually 
evolved  from  the  use  of  seedling  trees  to  a  point  where  the  grafted 
trees  were  probably  planted  in  the  majority  of  cases,  in  the  last  part 


BULLETIN    379]  WALNUT    CULTURE   IN    CALIFORNIA  17 

of  this  period.  During  this  time  there  was  a  rather  general  but  ill- 
founded  belief  that  the  grafted  trees  were  not  likely  to  be  as  long-lived, 
or  as  vigorous  as  the  seedling  trees.  This  belief  proved  to  be  entirely 
unfounded  on  fact,  and  by  the  end  of  another  decade,  or  by  1915, 
seedling  trees  were  no  longer  used. 

In  the  central  part  of  the  state  the  historical  development  of  the 
walnut  varieties  has  proceeded  quite  independently  of  the  southern 
part.  Although  there  were  hard-shell  trees  growing  in  many  places 
through  the  district,  little  or  no  use  was  made  of  their  progeny  in 
developing  the  present  day  varieties  of  central  California. 

Many  trees  and  scions  of  the  varieties  of  walnuts  commonly  grown 
in  France,  were  imported  into  California  in  the  early  seventies,  by 
the  late  Felix  Gillet  of  Nevada  City.  As  a  result  of  Mr.  Gillet  's  efforts, 
the  Franquette,  one  of  the  most  valuable  varieties  grown  in  France, 
became  the  leading  variety  of  central  California.  The  Mayette,  another 
French  variet}^,  took  a  somewhat  secondary  place  in  central  California 
by  the  same  means.  A  seedling  from  the  Gillet  nursery  produced  the 
original  Concord  tree  in  Contra  Costa  County,  and  from  this  tree  has 
developed,  by  grafting,  one  of  the  most  extensively  planted  varieties 
in  central  California.  During  the  past  ten  years,  the  Payne  variety 
has  steadily  been  gaining  friends  in  this  district.  For  a  detailed 
account  of  the  history  of  this  variety,  see  page  26. 

The  varieties  chosen  for  planting  should  be  those  most  marketable, 
most  productive,  and  best  adapted  for  growing  under  the  local  con- 
ditions in  question.  At  present  there  are  comparatively  few  varieties 
which  have  won  and  retained  their  popularity  with  the  growers  and 
the  marketing  trade.  Many  new  varieties  have  been  originated  in 
California  during  the  past  thirty  years,  but  most  of  them  have  been 
propagated  in  only  a  very  limited  way,  and  have  soon  fallen  into  dis- 
favor, owing  to  some  particular  shortcoming.  New  varieties  of  the 
future  should  be  a  decided  improvement,  in  some  particular,  upon 
our  present  sorts;  otherwise  they  are  not  worthy  of  even  limited 
propagation. 

The  principal  economic  advantages  which  a  grafted  or  budded 
grove  of  the  best  varieties  possesses,  in  comparison  with  a  seedling 
walnut  grove,  are:  (a)  the  budded  nuts  sell  for  a  higher  price  than 
the  seedlings,  since  they  are  more  uniform  and  of  superior  quality  ;* 


*  The  average  price  for  seedling  walnuts  sold  by  the  California  Walnut 
Growers'  Association,  during  the  15-year  period,  1909  to  1923,  inclusive,  is,  for 
No.  1  soft-shell  nuts,  19.5  cents  per  pound.  During  the  same  period,  Fancy 
Budded  nuts  averaged  22.4  cents  per  pound. 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

(6)  a  grafted  grove  is  more  uniform  and  has  fewer  unproductive 
trees  than  a  seedling  grove;  and  (c)  certain  varieties  are  less  suscep- 
tible to  blight  than  the  seedlings. 

There  is  no  one  best  walnut  variety  for  planting  in  the  whole  State 
of  California.  With  the  varying  soil  and  climatic  conditions,  the 
success  or  failure  of  a  variety  may  depend  upon  the  question  of  its 
adaptation  to  its  surroundings.  In  reading  the  discussion  of  the  varie- 
ties in  the  following  pages,  and  the  recommendations  which  are  made 
for  planting  in  the  several  localities,  it  should  be  borne  in  mind  that 
the  observations  concerning  the  varieties  have  been  very  general.  In 
many  instances  an  attempt  has  been  made  to  present  the  consensus  of 
the  opinions  of  several  observers,  such  as  walnut  growers,  packing 
house  experts,  and  nursery  men,  and,  although  the  varieties  described 
seem  to  be  the  best  for  commercial  planting  at  the  present  time,  they 
may  be  supplanted  in  the  future  by  varieties  entirely  unknown  at 
present,  or  by  varieties  which  may  not  be  highly  thought  of  at  this 
date.  The  selection  of  varieties  for  propagation  by  grafting  and  bud- 
ding has  gone  on  for  so  short  a  period  (20  to  30  years)  that  improve- 
ments may  be  confidently  expected  in  the  future. 

In  considering  the  varieties  to  plant  in  a  commercial  grove,  one 
should  select  only  those  which  meet  the  requirements  of  a  first-class 
commercial  nut  to  be  sold  in  the  so-called  "budded"  grade, f  or  one 
of  the  varieties  now  sold  in  the  special  grades  under  their  respective 
varietal  names.  It  is  a  decided  advantage  in  the  successful  sale  of  the 
crops  to  have  one  community  growing  only  one  or  at  least  only  a 
very  few  varieties,  so  that  a  uniform  product  can  be  sold  in  wholesale 
quantities. 

A  heavy  shell  and  a  firmly  sealed  nut  are  essential  to  withstand 
the  handling  in  the  commercial  grading  and  packing  operations,  with- 
out cracking  the  nuts  open.  A  nut  which  is  partly  cracked  open  soon 
becomes  rancid,  and  must,  therefore,  be  eliminated  from  first-class 
grades.  The  proportion  of  kernel  to  the  total  weight  in  a  carefully 
graded  sample  of  the  leading  varieties  of  nuts  is  slightly  less  than 
50  per  cent. 


t  At  present  only  a  portion  of  the  varieties  of  the  California  walnuts  are 
sold  at  wholesale  under  their  varietal  names.  The  Eureka,  Franquette,  Mayette, 
Payne,  <*•  nd  Concord  are  sold  as  such;  the  Placentia  and  Ehrhardt  which  com- 
prises a  g/eater  bulk  than  all  other  grafted  varieties  combined,  are  sold  under 
the  general  term  "budded." 


Bulletin  379]  WALNUT  CULTURE  IN  CALIFORNIA 


19 


Fig.  2. — Placentia  (natural  size). 


20  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

DESCRIPTION  OF  VARIETIES* 

(Described  in  order  of  their  popularity  in  1924) 

PLACENTIA 

(Generally  planted  in  southern  California) 

The  Placentia  may,  at  present,  be  considered  the  best  variety  of 
walnut  which  has  been  proved  thoroughly  successful,  commercially, 
in  southern  California.  At  present,  most  of  the  nuts  sold  in  the 
" budded"  grade  are  of  this  variety.  The  defects  of  this  variety  will 
be  stated  first,  and  its  good  qualities  later. 

The  Placentia  is  one  of  the  most  profitable  varieties.  During  1915 
and  1916,  however  when  blight  was  very  prevalent,  many  of  the 
Placentia  groves  blighted  as  badly  as  the  average  seedling  groves. 
Another  defect  of  this  nut  is  the  tendency  of  the  nuts  to  spring  open 
at  the  apex  if  they  are  dried  too  rapidly  by  exposure  in  the  sun  during 
the  curing  process. 

The  variety  is  readily  propagated  by  grafting  upon  the  black  wal- 
nut, making  a  thrifty,  rapidly  growing  tree.  The  Placentia  usually 
bears  young  in  southern  California,  and  produces  very  good  crops.  It 
is  regular  in  its  bearing  habits  and  lacks  the  tendency  of  some  sorts  to 
bear  every  other  year. 

The  nut  is  the  desirable  size  for  commercial  sale.  It  has  a  fairly 
smooth  shell  and  is  oval  though  the  nuts  tend  to  vary.  Some  strains 
are  nearly  round  and  somewhat  roughened.  In  an  attempt  to  learn 
the  cause  of  the  Placentia  being  composed  of  several  different  types, 
the  writer  during  the  winter  of  1924  visited  the  ranch  where  this 
variety  was  first  propagated.  In  interviewing  a  member  of  the  family 
who  was  a  mature  person  at  the  time  the  early  propagation  began,  it 
was  learned  there  were  originally  four  seedling  trees  from  which  this 
so-called  Placentia  Perfection  was  first  propagated.  Two-  of  these 
trees  furnished  more  scions  than  the  others  because  they  were  con- 
sidered somewhat  superior  as  regards  productivity,  and  nut  character- 
istics. No  attempt  was  ever  made  to  keep  the  scions  from  the  four 
trees  separate,  thus  this  so-called  variety  from  the  very  beginning  was 
really  composed  of  four  clonal  varieties  indiscriminately  mixed 
together.  It  is,  therefore,  not  surprising  that  many  writers,  during 
the  past  fifteen  years,  have  commented  upon  the  different  so-called 
types  of  Placentia.     It  may  well  be  expected  that  one  of  the  four 


*  For  a  more  complete  description  and  history  of  the  following  varieties,  as 
well  as  the  more  uncommon  ones,  see  Univ.  of  Calif.  Experiment  Station  Bull. 
No.  231,  1912,  and  U.  S.  Dept.  of  Agr.,  Bur.  of  Plant  Indus.,  Bull.  No.  254,  1913. 


Bulletin  379]  WALNUT  CULTURE  IN   CALIFORNIA  21 

original  seedlings  is  superior  to  the  rest  in  productivity  or  blight  resist- 
ance. There  are,  no  doubt,  many  heterogeneous  Placentia  groves  from 
which  scions  are  now  being  taken,  which  should  be  carefully  gone  over 
during  harvest,  to  eliminate  the  least  desirable  types.  The  two  out- 
standing types  most  commonly  found  in  the  Placentia  mixture,  are  a 
roundish  type  which  produces  quite  a  large  percentage  of  rather  rough 
nuts,  and  a  more  oval  type  which  is  composed  almost  exclusively  of 
very  smooth  nuts.  The  former  type  is  shown  in  the  illustration,  and 
is  probably  more  commonly  found  than  the  latter  type. 

The  nuts  are  not  well  sealed.  The  shell  is  thin,  but  strong.  The 
kernel  is  smooth,  plump,  and  light  colored,  and  is  taken  as  a  standard 
of  quality  and  appearance  for  the  budded  grade  of  nuts. 

The  Placentia  variety  has  been  grown  with  marked  success  through- 
out the  coastal  sections  of  southern  California,  and  some  of  the  best 
grafted  trees  in  the  inland  valleys  of  the  south  are  of  this  variety. 
The  Placentia  has  not  become  popular  with  the  walnut  growers  of 
central  or  northern  California,  because  it  seems  to  lose  some  of  its 
tendency  for  precocity  and  heavy  production,  and  is  further  hampered 
by  its  production  of  dark-meated  nuts  in  those  districts. 

EUEEKA*     S 

This  variety  was  originally  selected  because  of  its  supposed  resist- 
ance to  walnut  blight,  the  heavy  production  of  the  parent  tree,  and  the 
exceedingly  high  quality  of  the  nut.  The  supposed  resistance  to  blight 
has  not  been  substantiated,  f 

The  Eureka  is  not  usually  so  readily  propagated  upon  the  black 
walnut  root  as  some  of  the  Santa  Barbara  soft-shell  varieties.  The 
trees  are  vigorous  growers,  although  the  growth  may  not  be  so  great 
as  that  of  the  Placentias  in  the  same  orchard.  The  Eureka  is  later  in 
reaching  an  age  of  profitable  bearing  than  the  Santa  Barbara  soft- 
shell  varieties  in  southern  California,  and  although  the  high  quality 
of  the  Eureka  nuts  causes  them  to  sell  for  more  than  the  Placentia,  the 
difference  averages  only  about  2.9  cents  a  pound.  This  higher  price 
has  not  made  up  for  the  lighter  crops  which  are  obtained,  in  most 
instances  in  the  coastal  sections,  during  the  first  10  years'  growth  of 


*  The  parent  tree  of  the  original  Eureka  tree  was  a  Persian  or  Kaghazi  type 
of  walnut  which  grew  on  the  old  Meek  estate  near  Hayward,  California.  The 
seedling  which  developed  into  the  original  tree  was  located  in  the  dooryard  of  a 
ranch  about  a  mile  south  of  Fullerton.  Here  it  grew  to  be  a  mature  tree,  but 
died  of  a  root  disease  during  the  summer  of  1923. 

t  For  account  of  the  blight  occurrence  in  the  Eureka  walnut,  see  Fawcett 
and  Batchelor,  "An  Attempt  to  Control  Walnut  Blight,"  Monthly  Bulletin, 
Dept.  of  Agri.,  State  of  California,  May-June,  1920,  p.  177. 


22  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

the  trees.  Comparisons  made  between  15-year-old  trees  of  these  varie- 
ties indicate  that  the  Eureka  trees  will  eventually  become  as  produc- 
tive as  the  Placentia  in  the  coastal  areas,  and  probably  exceed  the 
Placentia  in  the  inland  valleys.  In  some  instances  the  Eureka  has 
seemed  to  have  a  tendency  toward  biennial  bearing  during-  its  first 
years  of  productive  age,  especially  in  the  coastal  areas.  In  the  warm, 
inland  valleys  this  variety  apparently  bears  as  regularly  as  any  other 
variety.  It  is  to  be  hoped  that  this  variety  may  prove  more  productive 
in  southern  California,  as  the  trees  become  15  to  20  years  old,  as  the 
nut  itself  has  many  points  to  commend  it,  and  large  acreages  have 
been  planted  to  this  variety. 

The  tree  blooms  much  later  than  most  other  varieties  and  often 
escapes  injury  to  the  flowers  by  late  spring  frosts.  Its  harvest  season 
is  fully  three  weeks  later  than  that  of  seedling  nuts,  or  of  the  Placentia. 
This  is  a  distinct  disadvantage  in  marketing  the  crop  in  the  eastern 
cities  in  time  for  the  holiday  trade. 

The  Eureka  variety  has  suffered  a  great  deal  from  injury  caused 
by  being  frosted  during  the  early  fall.  This  may  be  due  to  a  character- 
istic of  late  ripening,  or  to  the  possibility  that  the  Eureka  has  been 
planted  in  places  most  subject  to  frost  because  of  its  late  blooming 
habit. 

The  Eureka  is  less  susceptible  to  injury  from  sunburn  than  most 
varieties,  as  its  leaves  are  larger,  and  the  nuts  are  borne  in  a  position 
on  the  twigs  which  affords  more  natural  protection  by  the  shade  of  the 
leaves,  than  most  varieties.  This  does  not  make  the  Eureka  entirely 
immune  from  sunburn  injury,  however,  especially  if  the  hot  weather 
occurs  in  June  or  early  July,  when  the  nuts  are  only  half -grown,  and 
before  the  foliage  is  fully  developed. 

The  Eureka  nut  is  easily  distinguished  from  any  other  variety 
grown  in  southern  California  by  its  pronounced  elongation,  rather 
straight  parallel  sides,  slightly  rounding  to  square  ends,  with  the  apex 
usually  broader  and  more  nearly  square  than  the  base.  The  color  of  its 
shell  is  a  bright  straw,  of  a  lighter,  more  attractive  shade  than  the 
typical  soft-shell  nuts.  The  extra,  heavy  shell  and  the  exceptionally 
and  completely  firm  "seal"  of  the  nut,  make  it  a  model  nut  for  good 
keeping  qualities,  and  for  withstanding  commercial  handling.  A  well- 
grown  Eureka  nut  is  filled  to  its  full  capacity  with  a  light,  cream- 
colored,  plump,  waxy,  kernel,  with  rather  deep  convolutions,  possessing 
the  very  best  of  eating  qualities.  The  Eureka  is  richer  and  freer  from 
astringency  than  the  soft-shell  varieties. 


Bulletin  379]  WALNUT   CULTURE  IN   CALIFORNIA  23 


Fig.  3. — Eureka  (natural  size) 


24  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

This  variety  has  made  many  friends  in  central  California  around 
Stockton  and  San  Jose,  and  in  some  of  the  hot  inland  valleys  of  south- 
ern California,  where  it  seems  to  be  more  promising  for  profitable 
production  than  the  Placentia. 

The  kernel  of  the  Eureka  walnut  will  not  fill  out  perfectly  on  all 
four  quarters  unless  the  trees  are  amply  supplied  with  soil  moisture. 
The  variety  has  been  disappointing  in  some  of  the  central  California 
districts,  where  the  trees  have  not  been  adequately  supplied  with 
irrigation  water. 

On  the  merits  of  its  eating  quality  and  the  attractive  appearance 
of  its  kernel,  it  might  be  expected  to  out-sell  the  Placentia  more  than 
2.9  cents  a  pound ;  however,  this  average  figure  of  the  past  is  the  only 
safe  guide  in  projecting  a  business  undertaking.  It  is  to  be  hoped 
that  this  superior  nut  will  eventually  prove  to  have  the  tree  and  bearing 
characteristics  necessary  to  make  it  the  leading  commercial  variety  in 
the  interior  valleys  of  southern,  and  more  especially,  central  Califor- 
nia; the  evidence  at  hand,  however,  does  not  justify  an  unqualified 
confidence  that  such  hopes  will  be  realized. 

EHEHARDT    ^ 

This  variety  has  only  recently  been  planted  commercially.*  The 
location  of  the  original  seedling  tree  from  which  this  variety  sprang 
is  apparently  unknown. 

The  Ehrhardt  walnut  is  of  the  Santa  Barbara  soft-shell  type,  closely 
resembling  in  some  ways  the  Placentia.  It  is  somewhat  rougher, 
slightly  larger,  and  better  "sealed."  The  kernel  gives  the  impression 
of  being  more  plump  than  that  of  the  Placentia,  because  of  its 
convolutions  being  very  shallow. 

The  variety  is  readily  propagated  upon  the  black  walnut  root, 
making  a  tree  of  medium  vigor,  which  bears  at  an  early  age.  It  will 
probably  succeed  where  the  Santa  Barbara  soft-shell  succeeds.  After 
observing  the  original  trees  on  Mr.  V.  E.  Ehrhardt 's  place  at  Santa 
Ana  for  nine  years,  and  after  observing  the  results  obtained  by  top- 
grafting  trees  to  this  variety,  it  seems  reasonable  to  expect  that  the 
Ehrhardt  will  eventually  prove  its  superiority  to  the  Placentia  as  a 
heavy  yielding  variety.  The  Ehrhardt  trees  were  nearly  free  from 
blight  during  1915  and  1916,  when  this  disease  was  very  widespread. 
At  no  time  did  the  writer  observe  as  much  as  5  per  cent  of  blighted 


*  For  a  detailed  description  and  history  of  this  variety  the  reader  is  referred 
to,  Univ.  of  Calif.,  Bull.  No.  332,  p,  159. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


25 


Fig.  4. — Ehrhardt  (natural  size). 


26  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

nuts  on  the  Ehrhardt  trees,  while  Placentia  trees  in  the  same  orchard 
were  blighted  as  badly  as  45  per  cent,  and  seedlings  showed  65  per 
cent  by  actual  count.  Since  1916,  the  outbreaks  of  blight  have  not 
been  severe  enough  to  really  test  this  character  of  the  Ehrhardt.*  The 
Ehrhardt  should  not  be  considered  immune  to  blight,  but  as  far  as  it 
has  been  observed,  it  does  appear  more  resistant  to  this  disease  than 
most  varieties.  This  applies  only  to  the  disease  affecting  the  nuts, 
as  the  Ehrhardt  nursery  trees  are  not  particularly  resistant  to  blight. 
However,  whether  largely  resistant  to  blight  or  not,  its  tendency  to 
produce  very  heavy  crops  of  high-grade  nuts,  apparently  out-yielding 
the  Placentia,  makes  this  variety  worthy  of  extended  commercial 
planting,  especially  in  the  coastal  districts  of  southern  California. 
Several  trees  top-grafted  to  Ehrhardt  have  shown  remarkable  pro- 
duction in  Santa  Clara  County. 

The  nuts  are  considered  very  promising  by  the  wholesale  trade. 
They  easily  qualify  in  the  budded  grade.  One  of  the  distinct  advan- 
tages of  this  variety  is  the  fact  that  it  so  closely  resembles  the  Placentia, 
that  it  can  be  sold  with  this  latter  variety  in  the  "Budded"  grade. 
This  simple  wholesale  disposal  of  a  relatively  new  variety  is  a  decided 
asset,  if  it  is  to  be  planted  in  sections  already  largely  devoted  to  the 
Placentia. 

PAYNE    (V 

Although  the  parent  seedling  tree  of  this  variety  was  discovered 
by  G.  P.  Payne  in  his  grove,  near  Campbell,  California,  in  1898,  the 
variety  has  not  been  widely  planted  and  only  in  recent  years  has  it 
been  propagated  in  any  great  quantity.  This  variety  somewhat 
resembles  the  Franquette  regarding  shape  and  eating  qualities.  Its 
precocity  in  reaching  heavy  production  is  probably  the  chief  character- 
istic which  has  brought  it  into  prominence  during  the  past  few  years, 
and  in  this  respect  seems  to  exceed  any  other  variety  produced  in  the 
state.  The  Payne  is  notoriously  subject  to  blight  during  seasons  of  bad 
outbreaks.  The  recent  rise  in  popularity  of  this  variety,  may  be  very 
much  altered  if  walnut  blight  prevails  in  the  future  as  it  did  during 
the  seasons  of  1915  and  1916. 


*  No  explanation  can  be  offered  of  the  freedom  from  blight  of  the  Ehrhardt 
trees  during  1915  and  1916.  The  Placentia  trees  were  of  the  same  age,  or 
younger,  growing  in  the  same  orchard,  and  across  a  narrow  highway.  The  two 
varieties  bloom  at  practically  the  same  time,  and  may  be  artificially  inoculated 
with  blight  with  equal  ease. 


Bulletin    379]  WALNUT    CULTURE    IN    CALIFORNIA 


27 


Fig.  5. — Payne   (natural  size). 


28  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  Payne  tree  makes  a  rather  slow  growth,  possibly  because  of  its 
very  heavy  production  as  a  young"  tree.  The  nuts  are  borne  rather 
prominently  on  the  outside  of  the  trees  and  are  thus  somewhat  subject 
to  severe  sunburning  in  the  inland  valleys  during  years  of  abnormally 
high  temperatures. 

The  nut  is  oblong,  rather  pointed  at  the  apex;  shell  of  medium 
thickness,  somewhat  pitted  and  well  sealed ;  kernel  full,  with  moderate 
convolutions  and  of  exceptionally  mild  flavor.  With  the  heavy  pro- 
duction of  the  oldest  trees  growing  at  present,  there  seems  to  be  a 
reduction  in  the  size  of  the  nut,  making  a  high  percentage  of  No.  2 
grade.  This  nut  has  not  been  sold  in  the  Fancy  Budded  grade,  but 
has  been  sold  under  its  varietal  name.  It  is  well  liked  by  the  wholesale 
trade  and  usually  brings  l1/^  cents  a  pound  more  than  the  Budded 
grade,  on  account  of  its  especially  fine  quality. 

The  greatest  popularity  of  this  variety  is  found  in  the  Stockton, 
San  Jose,  and  Concord  districts.  In  the  Stockton  district,  it  is  as 
popular  as  the  Eureka.  It  might  be  suitable  to  interplant  in  a  Eureka 
grove  and  for  such  use  it  may  be  of  great  value.  Thus  the  first  ten 
years  of  the  slow-developing  productivity  of  the  Eureka  would  be 
bridged  over  by  this  prolific  and  precocious  variety.  In  the  districts 
of  the  coastal  counties  where  the  blight  is  most  regularly  severe  and 
prevalent,  the  Payne  variety  certainly  cannot  be  recommended  for 
permanent  trees. 

CONCOBD 

The  Concord  is  one  of  the  best  tested  varieties  for  the  central 
portions  of  California,  especially  in  Contra  Costa  and  Napa  counties, 
where  many  thousand  trees  are  growing.  In  these  districts  it  has 
proved  to  be  a  vigorous,  thrifty  tree,  and  an  annual  producer  of 
medium  sized  crops.  The  Concord  blooms  nearly  as  late  as  the  Eureka, 
but  possibly  the  nuts  mature  a  week  earlier  than  the  latter  variety. 
The  trees  are  fairly  precocious.  The  nuts  are  elongated  and  some- 
what pointed  at  the  apex;  the  shell  is  rather  smooth;  the  nuts  are 
poorly  sealed,  the  kernel  fairly  plump,  with  medium  to  deep  convolu- 
tions, only  medium  light-colored  and  of  good  quality.  This  varietj7 
is  not  so  well  received  by  the  wholesale  trade  as  the  Placentia,  Ehr- 
hardt,  Payne  or  Eureka.  It  is  doubtful  if  it  should  be  planted  where 
the  above-mentioned  varieties  will  succeed.  In  southern  California 
the  Concord  does  not  yield  so  well  as  the  soft-shell  varieties. 


Bulletin  379]  WALNUT   CULTURE  IN   CALIFORNIA 


29 


Fig.  6. — Grove  (natural  size). 


30  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

FRANQUETTE    ~  s 

This  variety  has  been  tested  longer  in  central  California  than  any 
other.  Successful  commercial  plantings  of  Franquette  occur  in 
Sonoma,  Napa,  Contra  Costa,  Santa  Clara,  Tulare  and  Kern  counties. 
It  has  many  adherents  in  these  sections  because  of  its  regular  crops 
of  moderate  size  and  of  excellent  quality.  It  is  less  liable  to  produce 
dark  kernels  due  to  high  maximum  temperatures  than  the  Santa 
Barbara  soft-shell  varieties.  The  Franquette  has  borne  too  lightly 
in  the  Stockton  district  to  be  recommended  for  commercial  use.  In 
southern  California,  the  Franquette  has  not  proved  commercially 
successful,  owing  to  its  slow  growth  and  light  crops. 

The  Franquette  is  late  in  reaching  a  bearing  age.  It  blooms  very 
late,  and  the  harvest  season  is  too  late  for  the  most  advantageous 
disposal  of  the  crop  for  the  eastern  holiday  trade.  Owing  to  its  late 
blooming  it  usually  escapes  injury  by  late  spring  frosts. 

The  nut  is  large,  elongated,  pointed,  fairly  smooth,  and  of  a  lighter, 
clearer  and  more  attractive  color  than  the  soft-shell  types.  The  shell 
is  thin,  but  well  sealed,  and  fairly  well  filled  with  a  very  light  colored, 
moderately  plump  kernel.  The  eating  qualities  of  the  Franquette  are 
unsurpassed  by  any  variety  grown  in  California.  The  high  quality 
of  this  nut  makes  it  sell  for  a  higher  price  than  most  other  varieties 
grown  in  the  state,  except  the  Eureka. 

Where  ample  irrigation  water  is  available,  the  Eureka  seems  to 
be  gaining  in  favor  more  rapidly  than  the  Franquette.  In  sections 
of  rather  heavy  winter  rainfall,  where  walnuts  are  usually  grown  by 
dry-farm  methods,  it  is  probable  that  the  Franquette  will  be  chosen, 
as  the  Eureka  may  not  produce  a  well-filled,  plump  kernel  under  such 
conditions.  The  heavy  weight  of  the  Eureka  nuts  may  make  them 
preferable  even  in  the  localities  where  the  Franquette  is  successfully 
grown,  if  the  soil  moisture  conditions  are  favorable  to  the  former 
variety. 

GROVE 
(Worthy  of  Trial) 

With  the  several  varieties  of  walnuts  now  available  for  planting  in 
California  and  their  suitability  to  the  natural  conditions  as  well  as 
the  demands  of  the  trade,  the  writer  is  very  loath  to  suggest  the  plant- 
ing of  any  untried  sorts.  For  the  simplicity  and  efficiency  in  the 
packing  and  sale  of  the  crop,  the  fewer  varieties  grown  the  better. 
At  the  same  time,  the  progress  which  has  been  made  in  the  use  of 
better  walnut  varieties  during  the  past  twenty  years  should  not  blind 


BULLETIN   379]  WALNUT    CULTURE   IN    CALIFORNIA  31 

us  against  the  possibility  of  further  progress  in  this  direction.  It  is 
with  this  principle  in  mind  that  the  variety  described  below  is  recom- 
mended for  limited  trial  in  the  various  districts  of  the  state. 

The  Grove  walnut  has  attracted  attention  throughout  the  neighbor- 
hood of  the  original  tree,  at  Lawrence,  California,  where  it  is  known 
locally  by  the  name  of  its  owner,  Mr.  L.  E.  Grove.  The  original  tree 
is  a  chance  seedling,  growing  in  the  dooryard,  and  it  was  one  year 
old  when  Mr.  Grove  came  into  possession  of  the  place.  Nothing  has 
been  learned  about  the  parentage  of  this  tree.  It  has  an  upright 
habit  of  growth  with  a  striking  amount  of  fruiting  wood  for  the  size 
of  the  tree. 

The  tree  blooms  late,  probably  a  little  earlier  than  the  Eureka, 
while  the  nuts  mature  from  October  1st  to  15th.  The  foliage  is  large 
and  dense  and  the  nuts  have  not  been  subject  to  sunburn.  Reduction 
of  the  crop  by  blight  injury  has  never  been  noticeable  on  this  tree. 
The  tree  and  nut  characteristics  suggest  that  this  tree  may  have  some 
Franquette  heritage. 

The  nut  is  of  medium  size,  oval,  somewhat  pointed  on  the  blossom 
end,  exceptionally  well  sealed,  with  a  heavy  shell.  The  kernel  is 
plump,  with  rather  deep  convolutions,  and  lighter  colored  than  the 
Santa  Barbara  soft-shell  type  of  nuts. 

The  percentage  of  kernel  in  several  random  samples  produced 
during  the  past  three  years  has  varied  from  50  to  52  per  cent.  From 
35  to  40  nuts  of  these  samples  weighed  a  pound. 

The  most  valuable  characteristics  of  this  variety  are  its  precocity 
and  heavy  bearing,  and  the  fine  eating  quality  of  its  nuts  which  are 
of  a  desirable  commercial  type. 

ROOTSTOCKS  AND  CHOICE  OF  NURSERY  TREES 
EOOTSTOCKS 
The  choice  of  rootstocks  for  the  walnut  has  narrowed  down  during 
the  past  20  years  to  a  predominating  preference  for  the  northern 
California  black  walnut  (J.  hindsii).  The  seed  is  shipped  commercially 
by  several  agencies  from  central  California  for  this  purpose.  Groups 
of  dooryard  and  border  trees  usually  furnish  this  seed.  Some  nursery- 
men in  southern  California  have  located  and  use  seed  from  particularly 
vigorous  black  walnuts  of  this  species,  which  have  been  planted  locally. 
The  seedlings  from  a  group  of  such  trees  vary  considerably,  and  in 
some  instances  certain  trees  are  especially  valued  because  of  their 
tendency  to  produce  uniform  and  vigorous  seedlings.  This  species 
of  black  walnut  will  apparently  withstand  a  more  unfavorable  soil 


32  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

condition  than  the  seedling  English  walnut.  Its  resistance  to  oak- 
root  fungus  (Armillaria  mellea)  is  also  a  valuable  characteristic.  The 
northern  California  black  walnut  is  more  vigorous  as  a  nursery  tree 
than  the  eastern  black  walnut,  which  was  formerly  used  to  some  extent, 
only  to  be  discarded  in  recent  years. 

The  southern  California  black  walnut  (J.  Californica)  should 
never  be  used  because  of  its  tendency  to  sucker  profusely  at  the  crown 
as  an  orchard  tree,  and  because  of  its  susceptibility  to  root-rot  troubles 
on  heavy  or  poorly  drained  soils. 

The  paradox-hybrid  walnut,  which  is  a  cross  between  the  English 
and  any  of  the  black  walnuts,  makes  a  remarkably  rapid  growing  tree. 
Mature  trees  may  have  a  spread  of  over  sixty  feet.  The  vigor  of  this 
first  generation  hybrid  has  been  mainly  responsible  for  its  popularity 
among  some  planters.  Such  trees  cannot  be  secured  in  quantity,  how- 
ever, so  that  their  use  will  always  be  restricted.  The  increased  vigor  of 
the  first-generation  hybrid  is  not  carried  on  to  following  generations, 
in  fact  the  first-generation  hybrid  trees  are  usually  nearly  barren 
and  their  seedlings  show  many  types  of  growth  and  vigor,  making 
undesirable  rootstocks. 

The  Royal-hybrid  walnut  is  a  cross  between  the  eastern  black 
walnut  and  the  California  black  walnut.  It  is  thought  by  some,  that 
the  second-generation  Royal-hybrid  is  equal  or  even  superior  to  the 
northern  black  walnut  as  a  rootstock.  There  may  be  such  hybrid 
strains  which  justify  this  confidence,  but,  as  a  whole,  it  cannot  be  said 
that  such  superiority  is  universal.  Seedlings  from  some  of  the  Royal- 
hybrid  trees  produce  uniformly  vigorous  trees,  while  those  from  others 
are  very  variable  and  lack  vigor.  Probably  the  pollinating  parent 
has  much  to  do  with  this  lack  of  consistency. 


METHOD  OF  PEOPAGATION 

The  English  walnut  is  usually  grafted  on  to  the  black  walnut  root 
during  the  early  spring,  using  a  whip  graft.  It  can  be  budded* 
however,  and  some  nurserymen  prefer  this  method. 


*  The  question  is  often  asked:  "What  is  a  budded  walnut?''  The  grade  of 
nuts  sold  under  the  name  "budded"  has  no  doubt  prompted  this  question  in 
many  instances.  A  budded  nut  is  any  one  of  the  varieties  which  will  grade 
according  to  the  standard  of  perfection  set  by  the  Placentia.  The  tree  which 
bears  such  a  nut  has  been  produced  by  budding  or  grafting  a  bud  or  scion  of 
the  variety  in  question  on  a  black  walnut  rootstock.  The  English,  or  the  hard- 
shell, is  seldom  used  as  a  rootstock  at  present.  If  budded,  the  work  is  usually 
done  in  August,  using  a  patch  or  a  shield  bud  from  the  current  season's  growth. 


Bulletin    379]  WALNUT    CULTURE    IN    CALIFORNIA 


33 


Fig.  7. — The  trees  in  this  orchard  were  planted  too  close,  being  spaced  40 
feet.  They  are  producing  walnuts  only  in  the  tops  and  will  soon  have  to  be 
thinned  in  order  to  obtain  a  satisfactory  yield  per  acre. 


Fig.  8. — An  old  seedling  grove  thinned  out  by  removing  every  other  diagonal  row. 


34  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Grafting. — One  year  from  the  time  of  planting  the  nuts  in  the 
nursery  the  black  walnut  seedlings  should  be  ready  for  grafting.  By 
this  time  they  should  have  attained  a  diameter  of  one  inch  or  more  at 
the  surface  of  the  ground.  The  surface  soil  is  hoed  away  from  the 
crown  of  the  trees  to  a  depth  of  2  or  3  inches  and  the  scion  is  inserted 
in  the  stock  just  below  the  surface  of  the  ground.  After  the  scion  is 
thoroughly  waxed,  the  soil  is  hoed  back  over  the  crown  of  the  tree 
covering  scion  and  all  to  a  depth  of  one  or  two  inches.  Well  pulverized 
soil  is  essential  in  covering  up  the  grafted  trees  as  the  presence  of  clods 
is  likely  to  result  in  the  scions  being  loosened  or  knocked  out  of  the 
stock. 

The  grafted  trees  are  grown  one  year,  and  trained  to  a  whip-like 
growth  free  from  lateral  branches.  It  is  necessary  to  tie  each  tree 
to  a  stake.  Stakes  1"  x  2"  by  8  feet  long  make  satisfactory  supports. 
Considerable  pains  is  necessary  throughout  the  growing  season  to  tie 
up  the  rapid  growing  trees  periodically  so  they  will  continue  to  grow 
straight. 

Budding. — When  walnuts  are  propagated  by  budding,  the  nuts 
are  given  an  early  start  and  the  seedlings  kept  rapidly  growing  until 
August.  At  this  season  they  are  large  enough  to  receive  a  bud  near 
the  surface  of  the  ground.  Patch  budding  is  the  method  most  com- 
monly followed  although  a  shield  bud  may  be  successfully  used  if  the 
"chip"  of  wood  is  carefully  removed  from  the  bud.  It  is  essential 
to  have  the  bud  "ripened"  by  removing  the  leaf,  leaving  the  petiole 
attached  to  the  base  of  the  bud  for  a  period  of  about  10  days  before 
the  buds  are  cut.  After  the  buds  are  inserted  in  the  stock  they  are 
firmly  tied  in  place  by  means  of  waxed  cloth.  This  must  be  loosened 
at  about  ten-day  intervals  until  the  buds  have  become  attached  to  the 
stock.  The  buds  are  usually  allowed  to  remain  dormant  until  the 
following  spring. 

Some  nurserymen  have  made  a  practice  of  growing  the  entire  tree 
(black -walnut  root  and  grafted  top)  in  one  year.  When  this  method 
is  followed  the  black  walnuts  must  be  cut  back  10  days  or  two  weeks 
after  budding  so  the  inserted  bud  will  be  forced  into  growth.  Larger 
and  more  desirable  trees  are  obtained  by  allowing  the  bud  to  remain 
dormant  until  spring  when  the  nursery  tree  is  cut  back  and  the  young 
budded  tree  trained  in  a  manner  similar  to  that  described  under  the 
heading  of  grafting. 


Bulletin  379]  WALNUT   CULTURE  IN   CALIFORNIA 


35 


Fig.  9. — Showing  scions  of  side  graft   (A)   and  bark  graft; 
(B)  in  place  ready  for  tying. 


36  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

REQUIREMENTS  FOR  GOOD  NURSERY  TREES 

Grafted  trees  sell  according  to  size,  a  premium  being  placed  upon 
the  larger  trees.  The  price  of  well-grown  walnut  trees  varies  from 
$1.00  to  $2.00.  A  medium  sized  tree  (8  to  10  feet)  may  be  preferable 
to  either  an  extremely  large,  or  a  small  tree.  A  very  large  tree  may 
be  injured  considerably  in  digging  from  the  nursery,  while  a  small 
one  may  be  stunted,  with  a  poor  root  system,  and  never  make  a  first- 
class  orchard  tree.  Although  a  considerable  premium  is  usually  paid 
for  the  largest  trees,  it  is  doubtful  if  they  are  actually  worth  any  more 
than  one  of  medium  size.  Several  first-class  young  groves  are  known 
to  the  writer  which  were  grown  from  medium  to  small  trees. 

STARTING  THE  YOUNG  ORCHARD 

PLANNING  THE  ARRANGEMENT  OF  THE  ORCHARD 

From  12  to  27  walnut  trees  to  the  acre  are  found  in  the  mature 
orchards.  General  observation  and  the  opinions  of  many  walnut 
growers  agree  that  the  trees  have  in  the  past  often  been  planted  too 
close. 

Orchards  planted  with  the  trees  60  feet  apart  each  way  are  among 
the  most  productive  in  the  state.  With  this  spacing  the  individual 
trees  have  room  to  develop  fully,  a  large  proportion  of  the  nuts  are 
produced  on  the  side  branches,  and  the  trees  maintain  a  healthy 
vigorous  growth  of  new  fruiting  wood  for  thirty  years  or  more. 

In  the  close  plantings,  where  the  trees  are  only  40  to  50  feet  apart, 
the  side  branches  are  shaded  most  of  the  day,  the  fruit  spurs  on  the 
lower  branches  soon  die,  and  the  crop  is  borne  mainly  in  the  tops  of 
the  trees.  A  closely  planted  grove  is  illustrated  by  figure  7.  Such 
orchards  have  not  maintained  their  productivity  so  well  as  those 
planted  less  densely.  Plantings  where  the  trees  are  40  or  45  feet  apart 
can  be  thinned  out  by  removing  every  other  tree  in  each  row  so  that 
the  trees  alternate,  or  stand  opposite  a  space  in  the  adjacent  rows. 
This  is  the  same  as  removing  every  other  diagonal  row.  The  remaining 
trees  will  then  stand  approximately  57  or  63  feet  apart  respectively 
in  the  rows,  running  diagonally  across  the  orchard.    See  fig.  8. 

One  of  the  most  favored  systems  at  present  is  to  plant  the  trees 
'M)  feet  apart  in  rows  which  are  60  feet  apart.  The  trees  may  be  all  of 
one  variety,  or  of  two,  planted  alternately.  With  the  latter  method, 
there  is  less  hazard  of  planting  the  wrong  sort,  as  one  has  a  choice 
between  two  varieties  when  the  time  arrives  for  removing  half  the 


Bulletin   379]  WALNUT   CULTURE  IN   CALIFORNIA  37 


Fig.  10. — Example  of  side  and  bark  graft  scions  tied  in  place  ready  for  waxing. 


38 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


trees.  Such  plantings  are  usually  thinned  out  when  the  trees  are  from 
10  to  14  years  old,  the  time  varying  according  to  the  variety,  soil, 
water  and  climatic  conditions.  They  should  not  be  left  until  they 
crowd  badly.  By  this  method,  properly  carried  out,  nearly  twice  the 
tonnage  may  be  expected  during  the  first  10  or  14  years  as  where 
only  the  permanent  trees  are  planted.  At  the  same  time,  there  is 
practically  as  much  room  for  intercrops  such  as  beans,  during  the 
early  life  of  the  orchard  as  where  only  12  trees  are  planted  to  the  acre. 
The  extra  expense  of  buying  and  planting  filler  trees  may  be  paid  for 
by  part  of  one  year's  crop,  while  all  other  extra  expense  due  to  the 
filler  trees,  such  as  pruning,  extra  water,  etc.,  is  if  minor  importance. 

CARE  OF  TREES  BEFORE  PLANTING 

If  the  trees  are  received  from  the  nursery  before  the  ground  is 
ready  for  planting,  they  should  be  unpacked  and  heeled  in,  where 


Fig.  11. — Scions  cut  for  side  graft  (A)  and  bark  graft  (B).  The  shape 
of  the  scion  may  make  it  necessary  to  reverse  the  position  of  the  cut  in  regard 
to  the  bud. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


39 


they  will  be  shaded.  The  soil  around  the  roots  should  be  thoroughly 
watered  after  they  are  heeled  in.  If  it  is  more  practical,  the  trees 
may  be  held  for  a  time  under  a  shed  with  the  roots  packed  in  damp 
sawdust  or  shavings. 

Just  before  planting,  the  roots  should  be  trimmed  by  cutting  off 
any  mutilated  portions. 

PLANTING  THE  YOUNG  OECHAED 

Nursery  Trees 

Walnut  trees  should  be  planted  during  January  or  February  so 
that  the  soil  may  thoroughly  settle  around  the  roots  and  growth  start 
with  the  beginning  of  the  normal  growing  season,  which  is  usually 
early  March,  in  southern  California.  Holes  should  be  dug  deep  enough 
to  allow  room  for  the  full  length  of  the  tap  root,  which  may  be  from 
18  to  30  inches.  The  lateral  roots  may  be  6  or  8  inches  long,  and  the 
hole  should  be  wide  enough  to  accommodate  them. 


Fig.  12. — Cleft  graft  ready  to  wax. 


40  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

In  filling  in  the  soil  around  the  roots,  it  is  advisable  to  use  the  top 
soil,  tamping  it  thoroughly  without  bruising  the  roots.  A  better  stand 
of  trees  may  be  expected,  and  a  more  promptTgrowth  in  the  spring,  if 
the  young  trees  are  thoroughly  irrigated  as  soon  as  they  are  planted. 
This  can  be  accomplished  by  running  a  single  furrow  along  the  row 
and  cutting  the  water  in  at  the  basin  left  around  each  tree  when  it 
was  planted.  Irrigation  at  this  time  may  not  be  essential  from  the 
point  of  view  of  soil  moisture  available,  but  it  will  be  worth  the  pains 
as  a  means  of  thoroughly  settling  the  soil  around  the  roots  to  prevent 
their  drying  out.  A  few  light  rains  should  not  mislead  the  planter 
and  cause  him  to  omit  this  first  irrigation  at  planting. 

Planting  the  Orchard  with  Black  Walnut  Trees  to  be  Top-Grafted 

In  several  parts  of  central  California,  especially  in  Contra  Costa 
County  the  planting  of  nursery  grown  walnut  trees  has  frequently 
been  followed  by  a  rather  poor  stand,  and  a  slow  growth  of  the  surviv- 
ing trees  during  the  first  2  to  3  years.  Walnuts  are  usually  grown 
in  this  district  without  irrigation  water  and  it  seems  probable  that 
the  poor  results  with  nursery  trees  is  directly  traceable  to  the  failure 
to  water  the  trees  at  the  time  they  are  planted.  In  any  event  the  past 
experiences  have  led  the  present  planters  to  favor  the  planting  of  the 
black  walnuts  in  the  field  and  top  grafting  the  resulting  trees  when 
they  are  from  2  to  5  years  old. 

Two  or  three  walnuts  are  planted  in  each  place  where  a  tree  is  to 
grow.  During  the  early  summer  the  smallest  trees  from  each  group 
are  pulled  out  leaving  the  most  vigorous  one. 

In  the  Stockton  and  Linden  districts  one-year-old  black  walnut 
trees  are  planted  in  orchard  form,  rather  than  nuts.  There  seems  no 
good  reason  for  planting  the  nuts  in  place  rather  than  the  trees  espe- 
cially if  irrigation  water  is  at  hand  so  the  trees  can  be  watered  at  the 
time  of  planting.  With  good  soil  conditions  and  water  available,  black 
walnut  trees,  or  nursery  grafted  trees  should  be  readily  transplanted 
with  nearly  all  making  a  good  growth  the  first  year. 

Various  methods  of  top-grafting  are  recommended  by  the  advocates 
of  this  general  method  of  starting  a  walnut  orchard.  The  two-year-old 
trees  may  be  grafted  in  the  main  limbs  near  the  trunk,  by  means  of  a 
side-graft.  If  the  trees  are  3  to  5  years  old  they  may  be  top-grafted 
by  means  of  a  bark  graft  or  a  cleft  graft.  Figures  9,  10,  11  and  12 
show  the  detail  method  of  making  side,  bark,  and  cleft  grafts.  What 
particular  type  of  graft  is  best  suited  to  this  method  of  propagation 
depends  largely  on  the  size  of  the  stock  and  the  scion.    The  side  graft 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


41 


is  preferable  on  small  branches,  and  the  bark,  or  cleft  graft  method  is 
the  most  useable  on  limbs  3  to  6  inches  in  diameter.* 


Fig.   13. — Black  walnut  top-worked  in  limbs. 


*  Whatever  method  of  grafting  is  practiced  one  of  the  most  important  details 
of  this  operation  is  in  regard  to  waxing.  All  cut  portions  of  the  stock  and 
scion  should  be  covered  with  wax  as  soon  as  the  scions  have  been  tied  in  place. 
A  hot  wax  made  from  the  following  formula  is  commonly  used:  resin  4  parts, 
beeswax  1  part,  linseed  oil  1  part.  There  are  also  prepared  waxes  on  the  market 
which  are  not  so  apt  to  crack  with  weathering  as  wax  made  according  to  the 
above  formula.  Bi-weekly  inspection  and  rewaxing  to  keep  all  the  cracks  in 
the  wax  covered  over  is  essential  to  success. 


42  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Patch  budding  is  another  method  some  times  followed  in  top- 
working  young,  black  walnut  trees.  Two-year-old  trees  with  one-year- 
old  frame  work  branches  %  to  %  of  an  inch  in  diameter  are  an  ideal 
size  to  top-work  by  patch  budding.  The  buds  are  usually  placed  in 
the  branches,  about  6  to  12  inches  from  the  main  trunk.  With  the 
buds  thus  placed  the  eventual  orchard  tree  has  a  trunk  and  the  base  of 
the  scaffold  limbs  of  the  black  walnut  as  shown  in  figure  13.  The 
black  walnut  wood  is  stronger  than  the  English  walnut  thus  there  is 
this  advantage  in  favor  of  such  top -worked  trees. 

Whatever  method  of  top-grafting  or  budding  is  used  considerable 
care  and  expense  will  be  necessary  in  bracing  and  tying  the  new  top. 
Without  adequate  bracing  the  scions  may  be  blown  out  of  the  stock 
before  they  reach  the  end  of  the  first  year's  growth.  It  is  not  advisable 
to  let  the  growth  go  beyond  12  or  15  inches  before  the  supports  are  put 
in  place  and  the  scions  receive  their  initial  tying.  Periodically  through 
the  first  year,  the  suckers  should  be  removed  from  the  trunk  and  frame- 
work branches  and  the  rapid  scion  growth  retied  as  the  growth  pro- 
ceeds. It  may  be  necessary  to  go  over  the  planting  four  or  five  times 
the  first  summer  if  the  trees  are  large  when  top -worked  and  the  growth 
of  the  new  top  rapid.  Figure  14  shows  the  method  of  bracing  the  top 
of  grafted  trees  during  the  first  year. 

Top  grafting  walnuts  is  an  art  which  requires  considerable  skill. 
The  follow-up  care  of  the  scions  the  first  year  requires  much  pains 
and  persistence.  This  method  of  establishing  an  orchard  is  less 
practicable  in  the  hands  of  most  farmers  than  planting  nursery  grown 
trees. 

TEAINING  YOUNG  TEEES 

If  walnut  trees  are  allowed  to  grow  without  any  pruning  they  will 
usually  take  their  natural  form  of  a  medium  tall,  upright,  pyramidal 
tree,  with  a  pronounced  central  leader.  At  the  time  of  planting 
the  trees  are  usually  headed  back  5  to  6  feet  from  the  ground,  and 
the  upper  lateral  bud  will  frequently  make  an  upright  limb  which 
will  take  the  place  of  the  removed  top  and  continue  the  leader  form 
of  tree.  If  this  central  branch  is  later  pruned  off,  however,  the  lower 
lateral  branches  will  form  a  more  open  vase-shaped  tree.  These  lower 
lateral  branches  thus  once  started,  take  all  the  strength  of  the  tree  and 
another  leader  seldom  becomes  established.  Figure  15  shows  an  eight- 
year-old  tree  growing  as  a  leader  type. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


43 


Fig.  14. — Supports  in  place  ready  to  tie  growing  scions  on  top-grafted  trees 
during  the  first  year. 


44 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Both  the  central  leader  and  the  open  vase  types  of  trees  have  their 
advocates.  The  advantage  of  the  leader  type  of  tree  is  primarily  in 
the  greater  strength  of  the  framework,  there  being  many  more  lateral 
limbs  distributed  along  a  greater  space  on  the  main  trunk  than  with 
the  vase-shaped  type,  and  thus  less  likelihood  of  breaking.     In  the 


Fig.  15. — Central  leader  type  of  walnut  tree.     This  type  has  a  strong  framework 
of  branches,  and  plenty  of  fruiting  wood. 


case  of  the  vase-shaped  type  the  few  branches  originate  at  nearly  the 
same  point  on  the  main  trunk  and  this  centralizes  the  strain  of  support- 
ing the  crop.  In  the  case  of  the  vase-shaped  trees,  it  is  not  unusual 
for  one-quarter  or  one-half  of  the  tree  to  break  off  and,  in  extreme 
cases,  there  may  be  a  split  down  the  middle  and  the  tree  be  entirely 
destroyed. 


Bulletin  379]  walnut  culture  in  California  45 

Figure  16  shows  an  old  walnut  tree  which  was  trained  to  the  open 
vase-shape.  It  may  be  necessary  to  have  from  15  to  20  per  cent  of  the 
trees  in  this  grove  braced  by  the  time  they  reach  an  age  of  profitable 
bearing.  The  example  here  illustrated  may  be  considered  extreme 
because  the  lateral  limbs  originate  at  so  nearly  the  same  point. 

After  one  has  decided  upon  the  type  or  ideal  toward  which  the 
young  trees  are  to  be  trained,  the  pruning  operations  should  be  con- 
sistent in  following  this  initial  decision.  In  training  the  young  walnut 
tree,  very  little  pruning  is  necessary.  Walnut  trees  as  they  are 
received  from  the  nursery  are  usually  one-year-old  whips.  If  vigorous 
8-  to  12-foot  trees  are  planted  they  are  usually  cut  back  to  within 
5  or  6  feet  of  the  ground. 


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Fig.  16. — Old  walnut  tree  trained  to  open  vase  shape. 

Trees  thus  planted  will  frequently  start  to  grow  first  from  the 
lower  buds  within  12  to  24  inches  of  the  ground,  or  will  send  out  shoots 
along  their  entire  length.  If  the  lower  lateral  buds  have  made  a  growth 
of  4  to  6  inches  it  is  advisable  to  ' '  pinch  off ' '  the  growing  tips  which 
will  have  the  effect  of  forcing  the  growth  into  the  upper  shoots  which 
are  to  form  the  eventual  framework  of  the  tree,  while  the  growth  of 
the  lower  pinched  shoots  is  restricted.  The  smaller  lower  shoots  thus 
produce  shade  and  nourishment  for  the  trunk.  This  growth  might 
be  rubbed  off  by  going  over  the  trees  frequently  during  the  early 
summer,  keeping  all  buds  off  which  occur  within  4  or  5  feet  from 
the  ground,  according  to  the  form  of  tree  to  be  grown,  and  their  height 


46 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


at  planting.  However,  the  former  method  is  preferable.  If  the  lower 
dormant  buds  are  rubbed  off  at  planting,  more  of  the  growth  will  take 
place  in  the  upper  buds  and  require  less  attention,  compared  with 
planting  them  without  this  precaution.  If,  on  the  contrary,  the 
lower  buds  are  allowed  to  grow,  the  buds  on  the  upper  24  inches  of 
the  tree  may  remain  dormant;  in  fact,  in  many  instances  the  upper 


ABC 

Fig.  17. — Walnut  trees  when  planted  are  usually  cut  back  to  within  5  or 
6  feet  of  the  ground. 

(A)  If  they  are  not  trained  they  will  frequently  send  out  shoots  along  their 
entire  length  during  the  early  summer. 

(B)  Frequently  if  the  lower  buds  within  two  feet  of  the  ground  are  allowed 
to  growj  the  upper  24  inches  of  the  trees  may  remain  dormant. 

(C)  If  the  lower  buds  are  suppressed  the  upper  ones  are  forced  into  growth 
and  the  framework  of  the  tree  is  started  in  the  upper  24  inches  of  the  trunk. 


Bulletin  379]  WALNUT  CULTURE  IN   CALIFORNIA  47 

portion  of  the  tree  dies  back  12  to  24  inches  following  the  vigorous 
growth  of  the  lower  shoots.  With  the  lower  buds  suppressed,  how- 
ever, the  upper  buds  are  forced  into  growth  and  the  framework  of 
the  tree  is  started  in  the  upper  24  inches  of  the  trunk.*  This  is  illus- 
trated by  figure  17. 

Whether  the  ideal  selected  is  the  leader  type  or  the  open  vase-shape, 
it  is  desirable  to  select  the  lateral  branches  for  the  framework  of  the 
tree,  spaced  as  far  apart  as  practicable  when  the  young  trees  are 
pruned  at  the  end  of  the  first  year's  growth. 

With  the  vase  type  this  perpendicular  spacing  throughout  18  to  24 
inches  will  give  greater  strength  to  the  framework  than  grouping  the 
branches  at  more  nearly  one  point  of  origin.  The  lateral  branches  to 
be  left  for  the  framework  of  the  tree  may  be  3  to  5  in  number,  as 
evenly  spaced  as  possible  as  they  radiate  outward  from  the  trunk.  The 
upper  or  second  bud  from  the  top  of  the  original  tree  will  usually  pro- 
duce a  branch  which  grows  in  a  nearly  upright  position;  this,  of 
course,  should  be  "pinched  back"  during  the  early  summer  or  cut 
out  during  the  dormant  season,  in  forming  the  framework  of  an  open 
vase-shaped  tree. 

In  the  case  of  the  leader  type  it  is  essential  to  space  the  laterals 
far  apart  and  not  leave  too  many;  otherwise  the  leader,  or  central 
shaft,  will  be  "choked"  out,  as  illustrated  by  figure  18.  Only  two  or 
three  of  the  laterals  should  be  left  in  addition  to  the  central  leader  at 
the  end  of  the  first  year,  as  other  lateral  branches  will  grow  out  during 
the  following  years  from  the  central  leader ;  thus  the  shape  of  the  tree 
is  formed  during  the  first  two  or  three  years'  growth.  The  branches 
which  are  left  on  the  tree  are  not  cut  back  in  common  practice  and  by 
no  means  should  the  leader  be  cut  back. 


*  A  few  planters  advocate  cutting  the  young  tree  back  to  within  12  to  16 
inches  of  the  ground.  This  method  forces  into  growth  the  latent  buds  near  the 
ground;  one  of  these  is  selected  to  make  the  tree,  and  all  others  are  rubbed  off. 
The  selected  shoot  must  be  staked  and  tied  to  a  2"  x  2"  stake,  5  to  6  feet  tall, 
otherwise  the  supple  growth  will  be  whipped  around  and  bent  over  by  the  winds. 
It  is  claimed  that  this  method  gives  a  better  stand  of  trees,  and  less  liability  of 
sunburn  injury  on  the  trunk.  The  tops  thus  cut  back  are  relatively  surer  to 
balance  up  with  the  root  systems  injured  by  digging.  It  may  be  well  to  doubt  if 
the  extra  expense  in  staking  and  in  the  care  necessary  to  grow  a  tree  in  this 
manner  will  repay  for  the  assurance  of  the  growth  of  a  larger  percentage  of  the 
transplanted  trees.  Good,  plump  nursery  trees,  if  handled  properly,  should 
transplant  very  readily  and  make  nearly  a  100  per  cent  stand  of  trees  without 
resorting  to  this  severe  cutting.  If  they  are  thoroughly  whitewashed,  or  use 
is  made  of  tree  protectors,  sunburning  need  not  be  feared.  It  may  be  advisable, 
however,  if  one  is  using  inferior  trees,  planting  very  late  in  the  spring  or  if 
using  trees  which  have  dried  out  in  transit,  to  cut  them  back  within  16  inches 
of  the  ground. 


48 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Fig.  18. — The  central  leader  in  this  tree  has  been  starved  out  by  the  lateral 
branches,  all  starting  out  at  nearly  the  same  place  and  cutting  off  the  food 
supply   from   the    central   trunk. 


VALUE  OF  ORCHARDS  AND  LANDS 
The  valuation  of  walnut  orchards  and  lands  suitable  for  walnut 
production  varies  considerably  in  the  different  localities  and  even  in 
the  same  districts.  Such  factors  as  suitability  of  the  soil,  water  rights, 
frost  conditions,  and  proximity  to  transportation  and  civic  centers 
are  reflected  in  the  appraised  valuation  of  walnut  properties.  The 
value  of  established  groves  may  be  greatly  influenced  by  the  variety 
of  nut,  rootstocks,  planting  arrangement,  etc.  The  average  or  the  most 
general  valuations  are  all  that  can  be  presented  here. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


49 


Replies  to  questionnaires  sent  out  by  the  California  Walnut 
Growers'  Association  in  1919  and  again  in  1921,  indicated  that  mature 
bearing  orchards  were  valued  at  about  $1400  an  acre,  while  the  young 
non-bearing  properties  were  valued  at  approximately  $900  an  acre. 
Table  4  shows  the  exact  average  valuation  and  the  number  of  properties 
considered. 

The  valuations  set  forth  in  table  4  are  large  compared  to  pre-war 
prices.  When  the  first  survey  quoted  here  was  made  in  1919  walnuts 
were  selling  at  the  highest  price  ever  realized  in  the  history  of  the 
industry.  In  1921  when  the  second  survey  was  made  the  price  had 
dropped  only  22  per  cent  whereas  many  farm  products  had  slumped 
to  their  pre-war  level.  If  the  price  of  walnuts  in  the  future  gradually 
approaches  pre-war  figures,  walnut  grove  prices  will  probably  decrease 
in  the  same  proportion,  but  with  some  what  of  a  "lag"  behind  the 
adjustments  in  price  of  the  crop.  For  the  trend  in  the  prices  of 
walnuts  see  table  1  on  page  4. 

TABLE  4 

Average  Valuation  of  Walnut  Groves 

(Including  Water  Eights) 


Year  Observations  were  made 

Valuation 

Number 'of 
properties 

Bearing 

Non-bearing 

represented 

1919* 

$1299.02 
$1477.05 

$995.00 
$326.65 

922 

19211 

231 

*  ' '  The  California  Walnut, ' '  p.  15,  1919.  Published  by  the  California  Walnut 
Growers'  Association. 

t  Brief  of  Facts  Eelating  to  the  American  Walnut  Industry,  p.  23,  1921. 
Published  by  the  Walnut  Protective  League. 

Land  suitable  to  walnut  culture,  plus  adequate  water  rights,  may 
be  purchased  at  from  $600  to  $1000  an  acre  in  the  proved  walnut- 
growing  sections  of  southern  California,  Such  land  may  have  a 
rather  fixed  value  for  the  production  of  beans,  citrus  fruits,  or  for 
civic  purposes;  thus  its  potential  value  for  walnut  production  is  only 
one  factor  which  determines  its  appraisement.  In  determining  the 
intrinsic  value  of  a  walnut  grove,  the  net  return  per  acre  for  a  period 
of  years  should  be  the  main  consideration. 

In  the  central  or  northern  portion  of  the  state,  where  the  walnut 
industry  is  not  so  extensively  developed  as  in  the  south,  land  which  is 
likely  to  prove  suitable  for  walnut  culture  may  be  purchased  for  $200 
to  $300  an  acre.  This  may  not  include  a  water  right  in  any  established 
irrigation  system,  but  usualy  such  lands  occur  in  areas  where  good 
wells  are  known  to  exist. 


50  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

CULTURE 

SOIL  MANAGEMENT 

There  is  no  one  best  way  to  cultivate  all  walnut  groves.  The 
methods  of  performing  the  various  operations  of  tilling  the  soil,  such 
as  plowing,  discing,  and  harrowing,  vary  widely  in  different  walnut 
groves.  These  different  methods  may  give  equally  good  results  in  their 
respective  groves,  when  measured  by  profitable  crop  production.  A 
practice  which  leaves  one  soil  in  first  class  condition  may  be  very  poor 
practice  on  soil  of  a  different  type.  Most  walnut  trees  are  deep-rooted 
and  it  seems  quite  apparent  in  many  cases  that  the  character  of  sur- 
face soil  cultivation  is  of  only  minor  importance  to  the  welfare  of  the 
tree,  except  as  this  cultivation  affects  the  conservation  and  use  of  soil 
moisture  by  the  tree.  There  are  many  dooryard  and  roadside  trees 
which  produce  heavy  crops  and  yet  the  soil  around  them  may  not  be 
stirred  from  one  year's  end  to  another.  It  does  not  follow  from  this, 
however,  that  a  walnut  grove  should  not  be  plowed  or  cultivated. 
There  are  frequently  many  conditions  surrounding  roadside  and  door- 
yard  trees  which  are  favorable  to  the  presence  of  an  abundance  of  soil 
moisture,  which  cannot  be  duplicated  under  orchard  conditions. 
Isolated  trees  have  the  advantages  of  better  exposure  to  sunlight  and 
a  far  greater  range  for  root  development  and  available  soil  moisture 
than  trees  planted  in  orchard  form. 

The  majority  of  walnut  groves  are  plowed  once  a  year,  usually  in 
the  early  spring.  The  practice  of  plowing  the  groves  seems  essential 
for  most  of  the  soil  types  found  in  the  walnut  districts  of  California. 
The  relation  of  such  plowing  to  the  penetration  of  the  rains  into  the 
subsoils  and  the  application  of  irrigation  water  alone  justifies  the 
operation.  Putting  6  to  10  inches  of  the  surface  soil  into  a  friable, 
mellow  condition  every  year  makes  it  practical  to  use  deep  irrigation 
furrows  and  thus  to  insure  deep  penetration  and  a  more  complete  use 
of  the  irrigation  water  applied  the  following  season.  An  annual  plow- 
ing prevents  the  tree  roots  from  becoming  permanently  established  in 
the  surface  six  inches  of  soil,  and  thus  becoming  subject  to  the  great 
variations  of  soil  moisture  content  which  occur  there  during  the  sum- 
mer months.  A  disc,  spring-tooth  or  spike-tooth  harrow  is  then  used 
to  put  the  land  into  good  physical  condition. 

If  the  land  is  plowed  in  the  fall  and  left  rough,  the  rains  will 
penetrate  deeper  into  the  subsoil  because  of  the  lessened  run-off.  This 
object  is  accomplished  best,  in  some  instances,  if  the  plowing  is  done 
at  right  angles  to  the  grade  of  the  land.    Where  a  winter  cover  crop  is 


Bulletin  379]  WALNUT  CULTURE  IN   CALIFORNIA  51 

grown,  fall  plowing  may  be  impractical  because  of  the  necessity  of 
planting  the  cover  crop  early,  either  just  before  the  harvest,  or  imme- 
diately after.  When  winter  cover  crops  are  not  grown,  the  practice  of 
fall  plowing  has  much  to  commend  it.  A  medium  to  light  irrigation 
(see  winter  irrigation,  p.  82)  may  well  precede  the  fall  plowing,  and 
the  land  left  rough  without  further  cultivation  until  the  early  spring. 

Late  spring  plowing  is  objected  to  by  some  walnut  growers  as 
they  believe  the  cutting  of  the  feeder  roots  by  such  plowing  to  be  espe- 
cially harmful  when  the  tops  are  just  starting  to  grow,  during  late 
March  and  the  month  of  April.  The  instances  which  are  frequently 
cited  to  illustrate  the  poor  crop  conditions  following  late  spring  plow- 
ing, are  not  readily  diagnosed.  It  usually  seems  that  there  may  be 
other  more  important  factors  conducive  to  poor  crops,  accompanying 
the  late  plowing.  The  dry  soil  condition  which  is  so  frequently  cor- 
related with  late  plowing  may  in  itself  be  more  truly  responsible  for 
the  poor  crops  than  the  mere  season  of  plowing.  Many  orchards  which 
are  plowed  late  are  cover-cropped  or  heavily  covered  with  volunteer 
growth  of  barley,  oats,  native  annual  plants,  etc.  In  most  seasons,  and 
in  most  walnut  districts,  the  presence  in  late  March  or  early  April  of 
a  sown  cover  crop  or  a  volunteer  crop,  may  be  taken  as  a  safe  indica- 
tion that  the  soil  moisture  is  largely  depleted,  unless  the  land  has 
been  thoroughly  irrigated  several  times  during  the  growth  of  the  crop. 
The  orchard  in  which  a  volunteer  crop  is  allowed  to  grow  is  most  likely 
to  be  neglected  in  regard  to  winter  irrigation.  Many  such  orchards 
are  plowed  in  the  spring  when  the  annual  plants  have  actually  wilted 
for  lack  of  moisture,  or  after  recent  rains  have  revived  them  by 
moistening  the  soil  only  to  the  depth  of  the  plow  furrow.  The  subsoil 
from  the  second  to  the  eighth  foot,  in  groves  thus  managed,  may  be 
nearly  dust  dry  at  this  season  when  the  trees  are  commencing  to 
leaf  out. 

The  amount  of  water  required  to  produce  a  cover  crop  is  often 
underestimated  by  the  walnut  grower.  This  is  especially  true  when 
the  crop  is  produced  during  the  rainy  season  and  makes  a  satisfactory 
growth  without  irrigation.  This  matter  is  more  fully  treated  under 
the  heading  "Cover  Crops."  Many  such  instances  of  the  above  con- 
ditions have  been  noted,  when  an  examination  of  the  subsoils  has 
shown  a  dryness*  hardly  credible.     The  fact  that  the  orchard  was 

*  Walnut  groves  have  been  examined  by  the  writer  in  which  the  soil  below 
the  penetration  of  winter  rains  contained  only  hygroscopic  moisture.  Walnut 
trees  commonly  reduce  the  soil  moisture  to  this  point  during  the  fall  months. 
A  winter  rainfall  of  8  to  12  inches  may  not  percolate  below  the  third  or  fourth 
foot  of  soil  if  there  is  a  cover  crop  or  weed  crop  growing  on  the  land.  If  the 
rains  come  in  small  amounts  distributed  over  five  or  six  months,  as  frequently 
occurs,  their  shallow  percolation  is  more  pronounced  than  when  large  volumes 
fall  at  a  time. 


52  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

plowed  in  the  early  spring  after  the  trees  had  started  to  grow  and  a 
few  feeder  roots  were  cut  has  seemed  unimportant,  compared  with 
the  extremely  unfavorable  soil  moisture  conditions  existing  at  the 
commencement  of  the  growing  season  for  the  trees.  It  seems  reasonably 
safe  to  assert  that  the  chief  factor  contributing  to  poor  crops  in  many 
late  plowed  orchards  is  the  dry  soil  condition  during  the  winter  and 
early  spring,  and  the  late  plowing  which  may  be  associated  with  this 
dryness  is  in  no  great  degree  responsible.  Late  plowing  may  be 
accompanied  by  careful  irrigation  practice  and  ideal  soil  moisture 
conditions  as  noted  later,  in  which  case  the  walnut  crops  may  not  suffer. 

Judging  the  question  of  late  spring  plowing  from  another  angle, 
there  are  many  first-class,  heavy  producing  orchards  which  are  anually 
spring-plowed  in  April,  when  the  leaves  are  coming  out  on  the  trees. 
Some  of  these  orchards  are  located  on  the  lowlands  subject  to  winter 
floods,  where  it  is  necessary  to  cover  crop  the  land  to  prevent  erosion 
and  where  this  threat  of  flood  damage  is  not  past  until  April.  Orchards 
in  these  districts  are  seldom  or  never  plowed  until  late,  yet  many  of 
them  yield  well.  Such  orchards  are  usually  well  irrigated  during  the 
spring  or  late  winter  months,  however,  either  by  regular  irrigation 
methods  or  by  the  passage  of  flood  waters  over  the  lands. 

This  discussion  should  not  be  taken  as  an  encouragement  to  late 
spring  plowing,  as  there  are  points  in  favor  of  plowing  early,  and 
it  seems  to  be  the  practice  most  usually  followed  by  successful  walnut 
growers.  It  is  offered  mainly  to  encourage  growers  to  feel  that  they 
may  maintain  a  first-class  walnut  grove,  even  though  for  any  reason 
they  are  obliged  to  plow  late,  provided  other  factors,  chiefly  soil 
moisture  conditions,  are  favorable  to  the  early  spring  growth  of  the 
tree  and  nuts  during  late  March  and  April  when  such  plowing  is 
frequently  done. 

The  clean  cultivation  during  the  summer  months  is  done  primarily 
in  relation  to  the  irrigation  practice  and  secondarily  for  the  conserva- 
tion of  soil  moisture  by  keeping  down  weed  growth  and  creating  a  dust 
mulch.  Preventing  weed  growth  is  of  much  more  importance  in  con- 
serving soil  moisture  than  the  creation  of  a  dust  mulch.  Weeds,  like 
any  other  type  of  plants  require  large  amounts  of  water  to  enable  them 
to  grow.  Deep-rooted  weeds  compete  for  soil  moisture  in  the  root  zone 
of  the  walnut  trees.  A  dust  mulch  on  the  other  hand  may  conserve 
soil  moisture  in  the  surface  soil  only.  When  the  soil  has  been  culti- 
vated sufficiently  to  leave  it  friable  to  a  sufficient  depth  to  facilitate 
irrigation,  further  cultivation  is  of  doubtful  value  unless  weeds  are 
being  destroyed. 


BULLETIN    379]  WALNUT    CULTURE   IN    CALIFORNIA  53 

The  practice  of  sub-soiling"  walnut  groves  may  be  justified  in 
relatively  rare  cases  where  it  is  difficult  to  apply  a  sufficient  amount  of 
irrigation  water,  because  of  the  formation  of  a  plow-sole.  Most  of 
the  soil  types  on  which  walnuts  are  growing  can  be  readily  irrigated  to 
a  depth  of  7  or  8  feet  without  resorting  to  sub-soiling.  The  eifect  of 
sub-soiling  on  impervious  soils  is  likely  to  be  only  very  temporary 
but  it  may  be  advisable  in  occasional  years  to  obtain  deep  penetration 
of  the  winter-irrigation  water.  Sub-soiling  should  be  done  when  the 
soil  is  dry;  soon  after  harvest  is  the  most  logical  time.  The  benefits 
derived  from  sub-soiling  under  conditions  where  the  practice  was 
justified,  has  led  to  the  wasteful  use  and  the  making  of  extravagant 
claims  in  favor  of  this  rather  costly  practice.  Land  which  can  be 
irrigated  at  the  rate  of  applying  2  or  3  acre-inches  per  acre  per  day, 
can  not  be  profitably  sub-soiled.  Even  some  of  the  heaviest  types  of 
soil  on  which  walnuts  are  planted,  such  as  the  Yolo  clay  loam  in  the 
Whittier  section  will  absorb  3  or  more  acre-inches  per  acre,  per  day 
without  cross  furrowing,  with  ordinary  cultural  practice,  combined 
with  annual  plowing.  Many  acres  of  this  type  as  well  as  sandy  loam 
soils  have  been  sub-soiled  in  the  past  probably  at  a  net  operating  loss 
to  the  owner.  Each  piece  of  land  must  be  judged  on  its  own  merits, 
regarding  the  irrigation  experience  thereon,  and  the  possible  benefits 
which  may  be  derived  by  sub-soiling. 


COVER  CEOPPING 

The  practice  of  cover  cropping  walnut  groves  prevails  in  probably 
somewhat  less  than  50  per  cent  of  the  total  area  of  bearing  groves. 
Many  groves  which  are  not  sown  to  cover  crops  produce  a  volunteer 
growth  of  various  annual  plants,  such  as  bur  clover,  lupins,  wild  oats, 
and  alfilerilla,  during  the  rainy  season. 

There  is  very  little  or  no  definite  information  concerning  the 
subject  of  the  effect  of  cover  crops  on  walnut  trees.* 


*  The  beneficial  effects  of  plowing  under  green  manure  crops,  as  measured 
by  the  crops  of  annual  plants  which  are  later  grown  on  the  same  ground,  cannot 
be  used  as  a  safe  guide  in  theorizing  on  the  effects  of  this  practice  on  walnut 
trees.  In  the  case  of  annuals,  there  is  no  competition  between  the  cover  crop 
and  the  primary  crop.  Even  with  annuals,  much  of  the  benefits  ascribed  to  the 
cover  crop  may  have  been  due  to  crop  rotation,  as  has  been  shown  by  various 
experiments  which  were  so  laid  out  as  to  measure  this  latter  factor.  Again, 
the  root  zone  of  many  of  the  annual  crops  which  have  been  used  to  indicate 
the  effect  of  cover  crops  upon  the  productivity  of  the  soil,  corresponds  more 
closely  to  the  root  zone  of  the  cover  crops  than  to  that  of  the  very  deep-rooted 
walnut  trees. 


54  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Though  we  have  no  definite  field  trials  to  inform  us  concerning  the 
effects  of  cover  crops  in  walnut  groves,  a  discussion  of  some  of  the 
factors  to  be  considered  may  be  useful.  Possibly  the  primary  factor  is 
the  problem  of  supplying  sufficient  irrigation  water  for  the  walnut 
trees  and  the  cover  crop  combined.  While  it  is  true  that  walnut  trees 
do  not  draw  very  heavily  upon  the  soil  moisture  during  the  winter 
months,  yet  it  is  highly  important  to  have  the  water  content  of  the 
soil  somewhere  near  optimum  during  the  entire  winter.  It  is  a  mistake 
to  allow  a  winter  cover  crop  to  deplete  the  soil  of  its  necessary  moisture. 
There  should  be  no  misconception  about  the  saving  of  soil  moisture 
because  of  the  shade  of  the  cover  crop.  This  shade  and  the  possible 
conservation  of  moisture  in  the  few  surface  inches  of  soil,  is  hardly 
worthy  of  mention,  compared  with  the  soil  moisture  necessary 
to  produce  the  cover  crop  itself. 

The  actual  water  required  to  produce  a  fair  crop  of  the  several 
plants  commonly  used  as  cover  crops  in  California,  has  never  been 
carefully  studied,  so  far  as  is  known  to  the  writer.  A  good  general 
idea,  however,  of  the  water  requirements  of  such  crops  can  be  obtained 
from  both  the  controlled  experiments  with  related  plants,  and  the 
experiences  of  farmers. 

In  studying  the  water  requirements  of  certain  plants,  Briggs  and 
Shantz*  found  that  different  plants  require  different  amounts  of  water 
to  produce  equal  quantities  of  dry  matter,  f 

The  average  requirement  of  alfalfa,  field  peas,  and  melilotus  alba, 
as  shown  by  table  5,  was  803  pounds,  which  is  more  than  is  needed  by 
common  grains. 

In  the  absence  of  any  more  information,  we  can  use  these  results 
to  estimate  the  approximate  water  requirements  of  the  cover  crops 
used  in  walnut  groves. 

Using  this  water  requirement  of  803  pounds  of  water  as  a  guide, 
and  a  yield  of  13  tons  per  acre  as  an  average  yield,!  with  20  per  cent 
dry  matter,  the  example  works  out  as  follows : 

5200  (lbs.  of  dry  matter)  X  803  (lbs,  of  water  required  per  lb. 
of  dry  matter)  ==  4,175,600  (lbs,  of  water  per  acre)  =  18.47 
acre-inches  per  acre  of  cover  crop. 


*  The  Water  Requirement  of  Plants,  U.  S.  Dept.  of  Agr.,  Bur.  of  Plant 
Indus.,  Bull.  No.  284,  1913,  p.  47. 

t  Dry  matter  is  that  portion  of  the  plant  which  remains  after  all  the  water 
has  been  evaporated  off  by  a  temperature  slightly  above  the  boiling  point  of 
water. 

$  W.  M.  Mertz,  Green  Manure  Crops  in  Southern  California,  Univ.  of  Calif. 
Agr.  Exp.  Sta.,  Bull.  No.  292,  1918,  p.  10. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


55 


This  figure  of  18.47  inches  is  based  upon  control  experiments  where 
the  crops  were  grown  in  cans  and  the  surface  evaporation  from  the  soil 
was  practically  eliminated,  whereas  under  orchard  conditions  there  is 
considerable  loss  from  surface  evaporation  while  the  young  cover  crop 
is  becoming  established  during  the  early  fall  months.  The  figure  is, 
therefore,  probably  too  low  for  orchard  conditions. 


TABLE  5 

Water  Eequirements,  Measurements  of  Crops  at  Akron,  Colorado 

(Pounds  of  water  necessary  to  produce  one  pound  of  dry  matter.) 


Crop 

Variety 

Water  Requiremen, 
(lbs.) 

Alfalfa 

Grimm 

1068 

Field  peas 

Canada 

800 

Sweet  Clover 

(M.  alba) 

709 

Average  of  three  legumes 

803 

Turning  now  to  a  practical  example :  a  cover  crop  of  yellow  sweet 
clover  (M.  indie  a)  was  grown  in  a  one-year-old  walnut  grove  on  the 
Citrus  Experiment  Station  grounds  during  the  winter  of  1920-21. 
The  land  was  cropped  to  barley  during  the  spring  of  1920,  and  there- 
fore contained  practically  no  soil  moisture  available  for  the  growth  of 
a  cover  crop  in  the  fall  of  the  same  year.  The  cover  crop  was  planted 
in  September,  1920,  and  plowed  under  in  April,  1921,  when  it  was  just 
past  full  bloom.  The  clover  growth  contained  some  volunteer  barley, 
so  that  it  closely  corresponded  to  the  cover  crops  in  the  flood  districts, 
where  barley  and  sweet  clover  are  sown  together.  From  the  date  of 
planting  until  the  middle  of  March,  15.8  acre-inches  of  irrigation  water 
per  acre  had  been  applied  to  this  crop.  At  this  time  the  clover  was 
just  starting  into  bloom  and  the  land  was  so  dry  that  it  could  not  be 
plowed  with  a  mould-board  plow  until  after  it  was  irrigated.  It  was 
then  irrigated  with  7  acre-inches  per  acre.  By  April  11  the  land  was 
again  too  dry  to  plow,  and  in  addition  to  the  irrigation  water  pre- 
viously given,  7.2  inches  of  rain  had  fallen  during  the  growth  of  the 
cover  crop,  making  a  total  of  30.0  acre-inches  per  acre  which  had  been 
used  by  the  small  walnut  trees  and  by  this  mixed  cover  crop  of  sweet 
clover  and  barley  of  maximum  tonnage  growing  to  maturity.* 


*  At  this  time  the  land  received  an  additional  7  acre-inches  per  acre  and 
was  then  plowed  as  soon  as  practicable;  the  last  7  inches  should  not  be  charged 
to  the  cover  crop,  as  it  was  largely  conserved  in  the  soil  by  prompt  plowing, 
leaving  the  soil  in  a  good  condition  for  the  young  trees  and  the  planting  of  a 
summer  intercrop  of  beans. 


56 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


If  the  crop  had  been  plowed  just  as  the  clover  came  into  bloom 
following  promptly  the  March  irrigation*  there  would  have  been  15.8 
acre-inches  per  acre  irrigation  water  and  7.2  inches  of  rain,  making 
23  inches  as  a  total  used  by  the  crop  up  to  this  period  in  its  growth, 
and  leaving  the  soil  quite  as  dry  as  it  was  the  fall  before,  f  The  stage 
of  development  of  the  crop  at  this  time,  just  beginning  to  bloom,  would 
have  been  the  ideal  condition  for  plowing  under.  The  tonnage  was 
nearly  at  its  maximum,  and  the  green  succulent  condition  of  the  stalks 
of  both  the  clover  and  the  barley  would  have  probably  assured  a  more 
rapid  disintegration  after  plowing  than  turning  it  under  in  April, 
when  the  flowering  season  was  practically  past. 


Fig.  19. — A  cover  crop  of  yellow  sweet  clover  (M.  indica)  ready  to  plow 
under  in  late  March. 


From  the  two  examples  cited,  it  seems  probable  that  the  growth  of 
a  legume  cover  crop  in  a  walnut  grove  should  be  expected  to  require 
from  20  to  24  acre-inches  per  acre  of  irrigation  water,  aside  from  the 
needs  of  the  trees.  The  winter  rains  alone  are  frequently  no  more 
than  sufficient  to  meet  the  requirements  of  the  trees  under  clean  cul- 
tural conditions.  In  practice  this  will  mean  from  three  to  four  heavy 
irrigations  during  the  winter  and  early  spring,  in  localities  receiving 
only  10  to  15  inches  of  winter  rainfall.  (See  "Irrigation.")  It  is 
sufficient  here  to  remind  the  reader  again  that  there  should  be  ample 
water  available  to  grow  the  winter  cover  crop,  and  that  after  the  plow- 


*  It  was  too  dry  to  plow  before  this  irrigation,  as  noted, 
t  The  crop  wilted  badly  before  the  March  irrigation  and  the  soil  was  judged 
to  be  between  the  wilting  point  and  the  hygroscopic  point. 


Bulletin   379]  WALNUT    CULTURE   IN    CALIFORNIA  57 

ing  in  of  the  crop,  the  walnut  trees  should  be  given  a  thorough  irriga- 
tion at  the  commencement  of  their  early  spring  growth.  If  the  require- 
ments of  the  cover  crop  alone  are  considered,  at  the  time  of  plowing 
the  subsoil  may  be  entirely  too  dry  to  promote  an  early  vigorous 
growth  of  the  trees  and  nuts,  even  though  the  surface  soil  is  moist 
enough  to  plow  with  a  mould-board  plow. 

One  of  the  benefits  derived  from  the  use  of  cover  crops  in  walnut 
groves  has  been  the  improvement  of  the  physical  condition  of  the 
heavy  soils  as  a  result  of  the  incorporation  of  large  amounts  of  organic 
matter.  The  experiences  of  many  observing  farmers  seem  to  agree 
that  the  clay  loams  are  made  more  friable  and  the  irrigation  water 
penetrates  them  much  more  readily  when  cover  cropping  is  practiced. 
The  latter  point,  with  reference  to  the  deeper  penetration  of  the 
irrigation  water,  may  alone  justify  the  use  of  cover  crops  on  certain 
soils. 

The  legume  cover  crops  most  commonly  grown  in  walnut  groves 
during  the  winter  months  are  bur  clover,  yellow  sweet  clover,  purple 
vetch,  and  horse  beans. 

The  seed  may  be  drilled  in,  or  sown  broadcast,  and  then  "brushed 
in"  or  harrowed  lightly  with  a  spike-tooth  harrow.  The  amount  of 
seed  required  per  acre  is  as  follows : 

Melilotus  indica  and  bur  clover 20-  30  lbs.  per  acre 

Vetch 70-  80  lbs.  per  acre 

Horse  beans 100-200  lbs.  per  acre 

It  is  essential  to  plant  the  cover  crop  as  early  in  the  fall  as  possible 
after  harvesting  the  nuts,  in  order  that  it  may  make  a  good  start  before 
the  cold  weather  commences,  and  thus  make  sufficient  tonnage  by  the 
middle  of  March  to  be  worth  plowing  under.  The  cover  crop  grows 
successfully,  sometimes,  if  sown  at  the  time  the  land  is  leveled  off  in 
preparation  for  the  harvest,  and  the  furrowing  out  and  irrigating  of 
the  land  postponed  until  the  harvesting  operations  are  over. 

Figure  19  shows  a  good  crop  of  yellow  sweet  clover  ready  to  turn 
under  in  late  March.  Cover  crops  are  occasionally  disced  under.  This 
is  not  so  desirable  as  deep  plowing,  because  it  does  not  incorporate  the 
organic  matter  in  the  soil  mass  so  close  to  the  tree  roots.  A  cover  crop 
disced  under  improves  the  physical  condition  of  the  surface  soil,  but 
does  not  supplement  the  plant  food  of  the  soil  so  much  as  if  plowed 
under. 


58  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

IRRIGATION 

Before  discussing  the  irrigation  of  any  crop,  it  is  well  to  under- 
stand the  extent  and  distribution  of  the  root  system  of  the  crop  in 
question.  It  is  a  commonly  held  opinion  that  walnut  trees  are  very 
deep-rooted  when  grown  on  the  majority  of  the  orchard  soils  of  south- 
ern California.  There  are  exceptions  to  this  general  rule,  but  in  the 
deep  friable  soils  of  the  typical  walnut  sections  the  problem  usually 
is  not  how  deep  do  the  roots  go,  but  where  do  they  stop  ?  In  treating 
the  matter  of  irrigation,  the  following  discussion  may  be  taken  to  apply 
to  the  volume  of  soil  included  in  the  first  eight  feet  from  the  surface 
of  the  ground.  The  discussion  is  confined  to  this  depth  of  eight  feet, 
not  because  it  is  judged  that  this  is  necessarily  the  absolute  limitation 
of  the  root  system,  but  because  observations  have  shown:  first,  that 
the  root-feeding  area  in  many  groves  extends  to  at  least  this  depth; 
second,  that  first-class  crops  can  be  grown  when  only  the  first  eight 
feet  of  soil  are  considered ;  and  third,  if  many  of  the  walnut  growers 
observed  and  frequently  examined  this  volume  of  soil,  they  might  be 
able  to  improve  their  irrigation  practice. 

Successful  walnut  production  in  southern  California  and  through- 
out the  San  Joaquin  valley  in  central  California  requires  close  atten- 
tion to  the  soil  moisture  content  of  the  soil  practically  the  year  around. 
In  these  districts  the  normal  rainfall  will  not  usually  penetrate  to  a 
depth  of  eight  feet  in  the  soils  most  frequently  found  in  walnut  groves. 
The  winter  rains  are  distributed  over  four  or  five  months,  and  usually 
come  in  relatively  light  showers.  This  condition  makes  the  relative 
loss  by  evaporation  rather  large  in  proportion  to  the  total  rainfall. 
Many  groves  have  growing  cover  crops  or  considerable  weed  growth 
in  them  during  the  winter,  which  continually  dries  out  the  surface  soil 
and  interferes  with  the  deep  penetration  of  the  rain.  Observations 
have  been  made  in  Riverside  County  for  a  number  of  years  which 
show  that  at  an  average  penetration  of  six  inches  of  soil  to  an  inch 
of  rainfall  is  a  fair  average  at  the  end  of  the  rainy  season,  in  most 
walnut  groves. 

If  the  soil  mass  throughout  the  root  zone  is  not  wet  at  the  end  of 
the  rainy  season,  and  at  the  beginning  of  the  growing  season  for  walnut 
trees,  the  available  irrigation  water  in  many  of  the  districts  is  insuffi- 
cient to  thoroughly  wet  this  volume  of  soil  after  the  rapid  reduction 
in  soil  moisture  commences  coincident  with  the  growth  of  the  trees. 

Even  in  the  many  walnut  groves  which  are  irrigated  during  the 
winter  time,  the  majority  of  the  acreage  is  not  thoroughly  wet  through- 
out the  deepest  portion  of  the  root  zone,  except  during  years  of  heavy 
rainfall. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


59 


The  relationship  between  adequate  soil  moisture  during  the  early 
spring  and  the  size  of  the  walnuts  produced  subsequently  is  clearly 
shown  by  a  study  of  the  annual  winter  rainfall  records  and  the 
percentage  of  number  one  nuts  produced. 


'00 

1     \ 

98 

1                     \ 
1                        \ 
1                          \ 

* 

A 

9b 

1           /    \        X 
1  /                    \       \ 

/  \ 

i   \ 

1    \ 

94 

NO.  /NUTS 

!     \ 

1     /v   1 

K                      \     \ 

92 

\      \ 

1            \ 
\ 
\ 

\ 

90 

5                            \ 
1                            \ 

i               \ 

\ 

\ 
\ 

\ 
\ 

/                             \/            ' 
/                                             / 

: 
\ 

\ 

88 

\ 
\ 
\ 

1    /\         \ 

/                                                  ; 
/                                                   ' 

\ 
\ 

\ 

i 

86 

1        \ 

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/      \         \ 

r              x          \ 
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* 

\       \      // 

\       \  /     / 
\                / 

\          \ 

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82 

\          \ 

i 

80 

\ 
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i                \    / 
/                  x^ 

/                                                   RAINFALL 

78 

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1 

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76 

19/6 

/$/<?                \ 

J               1920 
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J. 

/9  22 

12 


Fig.  20. — Curve  showing  yearly  percentage  of  No.  1  grade  seedling  nuts 
sold  by  the  California  Walnut  Growers'  Association  and  the  mean  winter  rain- 
fall from  December  to  March,  inclusive,  in  the  several  walnut  growing  districts. 

The  data  used  in  making  this  study  were  taken  from  the  rainfall 
records  during  December  to  March,  inclusive,  from  the  Weather 
Bureau  observation  stations  at  Santa  Barbara,  Los  Angeles,  Pomona, 
Riverside,  and  Tustin.  During  the  nine  rainy  seasons,  1914-15  to 
1922-23,  inclusive,  the  average  rainfall  at  the  above  stations  has  varied 
from  63  per  cent  of  normal  to  167  per  cent.  During  this  same  time 
the  percentage  of  number  one  seedling  nuts  (for  details  of  grading 
nuts  see  page  88)  has  varied  from  77  per  cent  to  86  per  cent.  The 
tendency  for  the  years  of  heavy  rainfall  to  be  followed  by  a  production 
of  a  large  percentage  of  big  nuts  is  shown  by  figure  20. 


60  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Although  the  curves  may  not  be  considered  parallel,  they  do  tend 
to  move  in  the  same  direction.  There  are  many  factors  besides  total 
winter  rainfall  which  operate  to  affect  the  soil  moisture  available  for 
tree  and  crop  growth  during  the  early  spring.  Among  these  are  dis- 
tribution, amount  per  storm,  weather  conditions  between  storms,  and 
season  of  rainfall,  together  with  the  demands  upon  the  moisture  by 
cover  crops  or  weed  growth.  These  may  all  bear  more  or  less  upon  the 
percolation  of  the  water  into  the  soil  and  thus,  finally,  the  amount 
available  to  the  walnut  tree.  Again,  many  walnut  growers  practice 
winter  irrigation.  In  such  cases,  the  volume  of  rain  may  not  be  a 
limiting  factor.  It  is  hardly  to  be  expected,  then,  that  the  lines  would 
be  parallel  during  the  entire  period ;  the  fact  that  they  tend  in  the  same 
direction  the  majority  of  the  seasons  represented  is  unmistakable. 

The  fact  that  the  lines  cross  between  1917  and  1918  may  be  due  to 
the  especial  effectiveness  of  the  rain  of  1917-18  season  in  percolating 
deeply  into  the  soil.  During  this  rainy  season,  the  rains  came  very 
late  and  extended  over  a  short  period  which  lessened  the  comparative 
loss  by  surface  evaporation.  Most  of  the  rain  fell  in  February  and 
March  and  thus  the  wTeed  growth  in  the  groves  was  not  started  until 
a  few  weeks  before  they  were  plowed,  this  also  abnormally  favored  the 
effectiveness  of  the  rains  in  promoting  the  growth  of  the  walnuts. 

The  chief  thing  that  can  be  learned  from  this  is  the  value  of  soil 
moisture  in  the  early  spring  in  producing  large  nuts.  If  this  is  a 
correct  analysis  of  the  facts  presented  the  practice  of  winter  irriga- 
tion of  walnut  groves,  which  is  followed  to  a  great  degree  in  some  dis- 
tricts, should  become  a  universal  practice  during  the  winters  of  scanty 
rainfall.  Winter  water  applied  at  the  rate  of  6  to  12  acre-inches  per 
acre  is  considered  essential  during  years  of  subnormal  rainfall.  With 
such  a  demand  for  water  in  any  walnut  growing  district  many  of  the 
groves  must  be  watered  before  it  is  known  whether  the  season  will 
prove  to  be  relatively  ''dry"  or  not,  otherwise  if  every  grower  post- 
pones winter  irrigation  until  its  absolute  need  is  evident  the  demands 
on  the  water  systems  will  exceed  the  supply.  The  use  of  winter  irriga- 
tion water  to  supplement  the  seasonal  rainfall  may  be  looked  upon  as 
an  insurance  policy  against  winter  drought  and  the  resultant  small 
percentage  of  No.  1  walnuts. 

Even  though  occasional  years  occur  when  the  total  winter  rainfall 
is  sufficient,  a  study  of  available  weather  records  show  that  the  extra 
expense  of  irrigation  will  probably  pay  big  returns  about  five  times 
out  of  nine,  and  will  be  profitable  to  some  degree  about  nine  in  every 
eleven  years.  This  certainly  justifies  the  practice  as  insurance.  (For 
a  discussion  of  the  sizes  and  prices  of  walnuts,  see  page  89.) 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


61 


In  discussing  the  practical  irrigation  of  walnut  trees  throughout 
the  growing  season,  perhaps  the  subject  can  be  most  clearly  presented 
if  the  water  requirements  of  the  crop  are  considered  from  the  begin- 
ning of  the  growing  season  in  the  spring,  through  the  summer  and 
harvest  seasons.  Unlike  most  fruit  crops,  the  walnut  is  grown  for  the 
seed  of  the  plant  instead  of  for  any  edible,  fleshy  portion  which  sur- 
rounds the  seed.  Experience  in  growing  stone  fruits,  such  as  peaches, 
shows  that  the  size  of  the  fruit  may  be  greatly  influenced  by  irrigation 
water  applied  late  in  the  seasonal  growth  of  the  fruit,  long  after  the 
pit  has  hardened.  This  late  increase  in  the  size  of  the  fruit  is  caused 
by  the  increased  thickness  of  the  fleshy  or  edible  portion  of  the  fruit, 
without  any  increase  in  the  size  of  the  pit  itself.  The  growth  of  the 
walnut  compares  with  the  growth  of  the  pits  of  the  stone  fruits,  all  of 
which  reach  their  full  size  long  before  harvest  and  before  the  shell 
is  fully  hardened.  The  walnut  shells  of  most  of  the  varieties  begin  to 
harden  about  the  middle  of  June. 


to 


J^^- 

o 

o 

I 
% 

1 
1 

NUT 

o 

o 

<    // 

MAY 

21 

31 

to 

JUNE       2 0 

30 

JULY 

/o 

Fig.  21. — Curve  showing  mean  weekly  growth  of  Placentia  walnuts  from 
May  3  to  July  10,  1923.  (Expressed  as  the  area  of  the  cross  section  which  is  in 
direct  relation  to  volume.) 


62  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  most  rapid  growth  of  the  walnuts  takes  place  during  the  five 
or  six  weeks  after  the  blossoming  period.  It  has  been  shown  by  field 
trials  and  by  casual  observation,  that  a  shortage  of  soil  moisture  in  a 
walnut  grove  during  the  early  spring  will  be  followed  by  a  production 
of  a  large  percentage  of  small  walnuts.  A  study  of  the  growth  of 
Placentia  walnuts  was  made  during  the  1923  season  the  results  of 
which  are  shown  graphically  by  figure  21.  The  production  of  a  large 
percentage  of  No.  1  nuts  is  so  essential  to  successful  walnut  culture 
that  the  details  of  the  observations  set  forth  in  figure  21  may  well  be 
included  in  a  consideration  of  the  irrigation  needs  of  this  crop.  Just 
when  this  period  of  rapid  growth  takes  place  and  when  it  ends,  is  of 
prime  importance  to  the  practical  walnut  grower.  At  the  end  of  the 
rapid  growth  period  ' '  the  die  is  cast ' '  so  far  as  the  size  of  the  walnut 
then  on  the  tree  is  concerned.  No  amount  of  mid-summer  or  late  irri- 
gation water  will  make  a  particle  of  difference  in  the  size  of  walnuts 
after  the  shell  begins  to  harden  so  that  it  is  cut  with  difficulty  with 
a  knife. 

In  obtaining  the  data  presented  in  figure  21  random  samples  of 
100  nuts  were  picked  weekly  in  a  similar,  careful,  systematic  manner 
from  a  group  of  Placentia  trees.  These  nuts  were  cut  across  their 
largest  diameter  between  the  stem  and  blossom  end.  Records  were 
kept  of  the  average  size  of  the  nuts,  as  well  as  of  the  average  size  of 
nuts  and  shuck  combined,  for  the  growing  season. 

The  actual  diameter  measurements  have  been  transformed  into 
cross  section  measurements  to  show  more  clearly  the  course  of  growth. 

The  curve  shows  the  relative  growth  from  May  3rd  to  July  12th. 
The  trees  bloomed  10  to  14  days  before  the  observations  commenced, 
at  which  time  the  small  nuts  were  1.2  centimeters  (%6  of  an  inch)  in 
diameter.  A  glance  at  the  curve  shows  that  the  rapid  increase  in  size 
of  the  nuts  took  place  from  May  3rd  to  May  31st.  After  May  31st 
the  curve  flattens  out  and  only  a  small  growth  is  made  up  to  June  28th. 
After  June  28th  the  differences  in  average  size  of  all  the  samples 
measured  were  within  the  range  of  probable  error  in  random  sampling, 
and  therefore  show  no  increase  in  size  after  that  date.  The  shuck, 
however,  which  corresponds  to  the  edible  portion  of  a  stone  fruit,  made 
more  relatively  late  growth  than  the  nut:  the  growth  curve  for  the 
shuck  did  not  flatten  out  much  until  the  last  of  June.  By  June  14th 
the  average  size  of  the  sample  was  equivalent  to  the  minimum  size  of 
a  Fancy  Budded  Nut. 

By  again  referring  to  the  curve  it  may  be  clearly  seen  how  little 
actual  growth  took  place  after  this  size  was  reached. 


BULLETIN    379]  WALNUT    CULTURE   IN    CALIFORNIA  63 

For  those  not  familiar  with  the  graphical  presentation  of  such  data 
it  may  be  added  that  the  cross  section  size  of  the  samples  of  Placentia 
walnuts  herein  described  increased  483%  during  the  28  days  from 
May  3rd  to  May  31st;  and  increased  only  18%  during  the  following 
28  days. 

It  must  be  clear  from  these  figures,  that  any  irrigation  water  which 
is  to  affect  the  size  of  walnuts  should  be  in  the  ground  during  the 
month  of  May,  and  the  earlier  it  is  applied  during  the  month,  the 
more  effective  it  will  be. 

The  winter  irrigation  serves  also  to  supply  needed  early  spring 
moisture  for  the  growth  of  the  trees.  Aside  from  the  small  amount  of 
moisture  transpired  by  the  dormant  trees  and  lost  through  the  surface 
evaporation  of  the  soil,  the  big  bulk  of  the  winter  irrigation  water  is 
available  for  the  tree's  use  in  the  spring,  provided  it  is  not  largely 
used  by  cover  crops  or  weeds  before  the  trees  begin  to  grow.  Observa- 
tions of  the  soil  present  by  means  of  a  soil  auger  or  soil  tube  which  will 
reach  to  depths  of  7  or  8  feet,  should  at  least  begin  at  the  time  the 
trees  start  to  leaf  out.  If  the  winter  rains  plus  the  winter  irrigation 
do  not  percolate  to  a  depth  of  eight  feet  during  the  winter  and  late 
spring,  it  is  advisable  to  apply  the  first  spring  irrigation  sometime 
in  April.  At  this  season  of  the  year  the  trees  are  beginning  to  leaf  out- 
and  the  young  nuts  should  soon  be  rapidly  increasing  in  size.  Filling 
the  subsoil  with  moisture  at  this  season  makes  a  reservoir  for  the  tree 
roots  to  draw  upon  during  the  early  part  of  the  summer,  while  the 
nuts  are  reaching  their  full  size.* 

The  soil  moisture  due  to  early  spring  irrigation,  winter  rains,  and 
winter  irrigation,  should  be  supplemented  by  water  applied  in  mid- 
summer to  promote  the  filling  of  the  nuts  with  plump  kernels.  This 
may  require  three  applications  of  water  in  June,  July  and  August, 
respectively,  or  possibly  only  the  first  and  last  month  mentioned, 
according  to  the  type  of  soil  and  the  climatic  conditions,  The  light 
sandy  soils  will  require  more  frequent  irrigation  than  the  heavier  soils, 
while  the  walnut  groves  in  the  hot  inland  valleys  will  require  more 
water  than  those  on  the  coast,  which  are  frequently  bathed  in  heavy 
fogs  and  where  the  daily  temperature  is  relatively  low.      Detailed 

*  It  should  not  be  inferred  from  this  that  soil  moisture  which  penetrates 
below  the  root  zone  is  drawn  upward  into  the  root  zone  by  capillary  action  as 
the  growing  season  advances.  Such  capillary  action  is  probably  negligible 
unless  a  water  table  is  present.  On  the  other  hand  the  tree  roots  are  less 
numerous  in  the  lower  root  zone  from  4  to  8  feet  from  the  surface  than  in  the 
surface  4  feet  of  soil.  Experiments  have  shown  that  the  water  is  more  slowly 
removed  from  the  lower  root  zone  and  may  persist  to  a  marked  degree  above 
the  wilting  point,  as  late  as  the  middle  of  the  summer,  when  the  upper  root  zone 
has  dried  to  a  point  where  irrigation  is  decidedly  needed. 


64  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

directions  for  the  irrigation  of  walnut  groves  cannot  be  presented  in  a 
general  treatment  of  an  industry  which  is  found  under  such  widely 
divergent  conditions.  Emphasis  should  be  laid,  however,  upon  the 
need  of  a  continuous  supply  of  soil  moisture  from  the  time  the  tree 
leafs  out  in  the  spring,  up  to  and  including  the  beginning  of  the 
harvest  period.  Just  as  the  early  spring  moisture  is  essential  to  the 
production  of  large  nuts,  and  the  midsummer  water  to  the  full  develop- 
ment of  the  kernels,  ample  soil  moisture  in  early  September  is  neces- 
sary to  promote  the  normal  development  of  the  nut  to  the  point  where 
the  husk  cracks  open  and  the  nuts  drops  free  to  the  ground,  leaving  the 
husk  temporarily  attached  to  the  twigs.  The  absence  of  sufficient 
water  during  the  final  development  of  the  crop  is  almost  invariably 
associated  with  the  sticking  of  the  husks  to  the  nuts,  sunburning,  and 
a  consequent  high  percentage  of  cull  nuts.  The  shells  of  the  nuts 
which  crack  out  of  the  husks  naturally  are  mostly  of  a  bright  amber 
color  and  free  from  stains,  while  those  to  which  the  husk  adheres  are 
often  stained,  and  may  even  be  classified  as  culls,  if  the  stain  is  too 
deep  to  be  obliterated  by  the  bleaching  at  the  packing  house. 

It  should  therefore  be  the  aim  of  the  walnut  grower  to  keep  all  the 
soil  from  the  surface  to  a  depth  of  eight  feet  well  supplied  with 
-moisture  from  early  spring  until  early  fall. 

It  is  a  common  observation  of  practical  walnut  growers  that  it  is 
more  difficult  to  obtain  deep  percolation  of  the  irrigation  waters  in 
midsummer  than  in  spring.  This  condition  may  be  largely  due  to  the 
fact  that  subsoils  have  not  been  thoroughly  moistened  in  the  early 
spring  and  are  allowed  to  become  too  dry  before  irrigation  in  mid- 
summer. This  excessively  dry  soil  resists  the  entrance  of  moisture  to 
a  much  greater  degree  than  a  soil  which  is  moderately  damp  at  the 
time  irrigation  water  is  applied.  This  phenomenon  of  the  movement 
of  soil  moisture  into  soils  of  different  moisture  contents  has  been 
observed  often,  both  in  the  laboratory  and  the  field.  Figure  22  illus- 
trates an  orchard  being  irrigated  by  the  dike  and  check-furrow  system. 
This  prevents  "run-off"  when  a  large  volume  of  water  is  used  and 
secures  deep  penetration  of  moisture. 

Just  when  the  time  arrives  to  commence  spring  irrigation  and  when 
more  water  should  be  applied  in  mid-summer  depends  upon  the  cul- 
tural and  irrigation  practice  which  has  prevailed  during  the  winter 
and  early  spring.  If  the  land  has  not  been  winter  irrigated,  or  if  a 
cover  crop  has  been  grown  without  adequate  water,  the  subsoil  may 
already  be  dried  out  below  the  wilting  point  by  April  1.  It  is  impos- 
sible to  lay  down  any  hard  and  fast  general  rule  that  can  be  followed 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


65 


without  examining  the  soil.  Every  walnut  grower  should  study  and 
frequently  observe  the  moisture  condition  of  the  subsoil  as  well  as  the 
surface  soil  in  the  walnut  grove.  Groves  which  are  served  by  pumping 
plants  may  be  irrigated  as  needed.  For  those  which  depend  upon  a 
mutual  gravity  water  system  it  is  necessary  to  arrange  the  water 
schedule  in  advance  on  the  basis  of  close  observation  of  their  needs  in 
the  past. 


Fig.  22. — This  system  of  the  dike  and  check  furrow  irrigation  is  used  to 
prevent  "run  off"  and  obtain  a  deep  moisture  penetration. 

The  subsoils  can  be  examined  and  readily  sampled  by  means  of  a 
soil  auger.  Such  a  tool  can  be  made  by  any  blacksmith  and  may 
consist  of  a  two-inch  carpenters'  auger,  welded  to  a  steel  rod,  or  a 
half-inch  gas  pipe.  For  the  intelligent  irrigation  of  a  deep-rooted 
crop  like  the  walnut,  a  soil  auger  is  as  much  a  necessary  part  of  the 
equipment  as  an  irrigators'  shovel.*  Opposite  sides  of  a  ten-acre  tract 
may  take  water  quite  differently  in  the  subsoil,  while  the  surface  soil 
seems  uniformly  irrigated.  Only  by  the  use  of  a  soil  auger  can  the 
farmer  make  any  more  than  a  poor  guess  at  the  soil  moisture  conditions 
throughout  the  deep -root  zone.  Figure  23  shows  a  farmer  examining 
the  subsoil  in  his  grove  to  a  depth  of  eight  feet. 

The  amount  of  water  necessary  to  wet  down  through  the  root  zone 
will  depend  largely  upon  the  type  of  soil  and  the  degree  of  dryness 
which  prevails  before  the  water  is  applied.  In  general,  it  will  require 
from  1*4  to  1%  acre-inches  per  acre  for  each  foot  depth  of  soil,  to 


*  A  soil  auger  has  been  designed  by  the  Orchard  Management  Division  and 
is  sold  to  walnut  growers  at  cost  by  the  California  Walnut  Growers'  Association. 


66 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


bring  the  moisture  from  the  wilting  point  or  below,  up  to  the  normal 
field  moisture-holding  capacity  of  the  soil.  This  may  be  taken  as  only 
a  very  general  rule,  however,  as  a  clay  loam  soil  which  has  been 
allowed  to  become  very  dry  may  require  more  than  this,  while  a  sandy 
loam  only  moderately  dried  will  require  less. 


Fig.  23. — A  farmer  examining  the  subsoil  in  his  grove  to  a  depth  of  8  feet. 


If,  for  example,  it  is  desired  to  apply  water  enough  on  a  sandy  loam 
soil  to  penetrate  to  eight  feet  from  the  surface,  it  may  require  10  acre- 
inches  per  acre  (l1/^  X  8  =  10)  if  the  subsoil,  as  well  as  the  surface 
soil  is  dry  at  the  beginning  of  the  irrigation  run.  On  the  other  hand, 
6  acre-inches  may  be  ample  if  the  subsoil  is  not  excessively  dried.  On 
land  underlaid  by  clay  or  gravel  at  less  than  eight  feet  from  the  sur- 
face, there  is  a  likelihood  that  the  root  zone  is  shallower  than  in  the 
preceding  example  and  correspondingly  less  water  will  be  needed. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


67 


If  the  distribution  of  the  root  system  of  a  crop  justifies  the  practice 
of  applying  water  sufficient  to  wet  down  to  a  depth  of  eight  feet,  such 
a  use  of  water  at  infrequent  intervals  will  be  more  economical  than 
frequent  shallow  irrigations.  In  the  former  case  a  smaller  percentage 
of  the  total  water  applied  will  be  lost  by  surface  evaporation  in  the 
first  eight  or  ten  inches  of  soil.  The  frequency  of  irrigations  desirable 
will  depend  upon  the  age  and  the  number  of  trees  per  acre,  the  soil 
type,  the  presence  of  intercrops,  climatic  conditions,  etc.,  all  of  which 
comes  back  again  to  the  necessity  of  each  farmer  ' '  going  to  the  bottom ' ' 
of  his  own  soil  conditions  with  a  soil  auger. 


Fig.  24. — If  the  land  is  check-furrowed  at  the  lower  half  of  the  orchard,  the 
run-off  will  be  reduced  and  the  added  pressure  of  deeper  water  in  the  furrow  will 
help  to  force  the  moisture  into  the  dry  subsoil. 


Measurement  of  the  water  applied  (explained  on  page  12)  is  as 
important  as  studying  its  movement  through  the  subsoils.  It  may 
prove  a  greater  mistake  in  the  end  to  apply  too  much  water  than  not  to 
apply  enough.  Water  applied  in  excess  is  lost  in  the  country  drainage 
and  may  contribute  to  the  rise  of  seepage  water,  to  the  eventual  detri- 
ment to  lands  lower  down,  if  not  to  the  land  in  question.  This  may 
seem  a  far  cry  to  many  people  who  own  groves  where  the  ground 
water  is  now  30  to  50  feet  from  the  surface.  Such  a  rise  of  ground  or 
seepage  waters  has  occurred,  however,  through  many  valleys  of  the 
West  where  the  ground  waters  were  formerly  30  to  40  feet  from  the 
surface.  There  are  literally  thousands  of  acres  of  good  walnut  land 
in  California  which  have  been  ruined  by  excessive  irrigation  of  higher 
lands.  Many  fine  groves  have  been  reduced  to  mediocrity  by  a  rise 
in  the  water  table  (see  drainage  of  walnut  groves,  page  10).    Much,  of 


68 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


course,  depends  upon  the  nature  of  the  deep  soil  formations.  It  is 
sufficient  now  to  say  that  the  far-sighted  intelligent  irrigator  who 
looks  upon  his  walnut  grove  as  a  permanent  investment,  will  study  the 
movement  of  soil  moisture,  not  only  throughout  the  root  zone,  but 
farther,  and  inquire  into  the  possible  loss  of  water  by  deep  percolation 
beyond  the  reach  of  the  walnut  trees. 

The  actual  method  of  applying  the  water  to  the  soil  must  be  adapted 
to  the  conditions  of  each  case.  Three  different  systems  of  applying 
water  are  illustrated  by  figs,  22,  24,  and  25.  The  advantages  of  the 
respective  method  are  briefly  stated  under  each  illustration. 


Fig. 


-If  the  water  will  move  into  the  soil  readily  the  use  of  deep  furrows 


and  the  wetting  of  as  little  of  the  surface  as  possible  will  reduce  the  moisture 
loss  by  evaporation  to  a  minimum. 


INTEKCBOPPING 

Intercropping  of  young  walnut  groves  is  the  practice  in  most  cases. 
This  practice  has  usually  proved  very  successful  in  making  the  land 
support  the  orchard  before  the  walnuts  come  into  profitable  bearing. 

The  best  intercrop  to  use  will  depend  somewhat  upon  local  circum- 
stances. In  the  bean  districts  and  in  cases  where  the  walnut  groves 
are  large  enough  to  justify  the  maintenance  of  bean  farm  machinery, 
beans  are  an  ideal  intercrop  for  the  young  grove.  The  bean  is  a  legume 
and  requires  thorough  cultivation.  These  facts  contribute  to  the  best 
conditions  for  the  development  of  the  young  trees,  if  sufficient  irriga- 
tion water  is  available  for  both  crops.  The  bean  straw  can  be  plowed 
under,  which  is  an  advantage  of  the  bean  intercrop  over  all  others. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


69 


Beans  may  be  grown  in  a  grove  until  the  trees  are  eight  or  ten 
years  old  as  shown  in  fig.  26. 


Fig.  26. — Showing  beans  used  as  a  profitable  intercrop  in  a  ten-year-old 
grove  in  a  bean-growing  district. 


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Fig.  27. — Illustrates  vegetable  intercrop  in  young  walnut  grove  which 
is  also  interset  with  peaches. 

Outside  the  bean  districts,  the  choice  of  intercrops  largely  depends 
upon  the  market  conditions  for  the  proposed  crops.  Several  of  the 
vegetables  commonly  grown  for  canning  factories  may  be  profitably 
grown  in  the  young  orchard  without  harm  to  the  trees.    Such  crops 


70 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


as  tomatoes,  peppers,  and  string  beans  are  often  used.  Figure  27 
illustrates  a  vegetable  intercrop  in  a  young  walnut  grove,  which  is 
also  interplanted  with  peaches. 


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Fig.  28. — Curve  showing  annual  average  production  per  tree  before  and  after 
intercropping  with  alfalfa,  compared  with  a  plot  which  has  been  continuously 
clean  cultivated. 

Intercrops  of  corn,  milo,  squashes,  and  pumpkins  are  not  so  well 
thought  of  among  many  experienced  walnut  growers,  because  of  their 
apparently  harmful  effect  upon  the  trees.  Milo,  especially,  is  generally 
considered  a  bad  intercrop  for  walnuts.  Milo  is  a  very  drought-resist- 
ant crop  with  a  relatively  low  water  requirement.  It  will  therefore 
develop  a  first-class  crop  under  soil  conditions  unfavorable  to  the  best 
growth  of  the  walnut  trees.  The  thrifty  appearance  of  the  milo  may 
deceive  the  farmer  concerning  the  condition  of  the  walnut  roots.  Only 
the  closest  students  of  irrigation  should  attempt  to  grow  this  crop  in  a 
walnut  grove,  and  it  may  be  of  doubtful  value  even  then. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


71 


The  use  of  alfalfa  as  an  intercrop  is  frequently  seen  in  groves 
which  appear  to  be  fairly  productive.  The  effect  of  this  crop  combina- 
tion has  been  studied  by  a  field  trial  in  Orange  County.  Figure  28 
shows  the  average  production  per  tree  during  two  years  before  and  five 
years  during  the  trial  on  a  three-acre  plot  intercropped  with  alfalfa, 
compared  with  an  adjacent  plot  of  the  same  size  which  was  clean  culti- 
vated summer  and  winter.  The  alfalfa  was  planted  during  the  fall  of 
1918.  The  yield  records  of  1917  and  1918  show  that  the  alfalfa  plot 
was  probably  the  most  productive  of  the  two  before  the  experiment. 


Fig.  29. — Alfalfa  intercrop  in  walnut  grove  covering  only  half  the 
ground  proved  deterimental  to  the  production  of  walnuts. 


During  the  five  years  of  the  experiment  the  production  of  the 
alfalfa  plot  has  been  on  an  average  67  pounds  (weight  at  harvest) 
per  tree  less  than  that  of  the  clean  culture  plot.  Allowing  for  shrink- 
age in  curing  of  15  per  cent  and  using  the  respective  current  year's 
prices  of  nuts  as  a  basis  of  calculation  a  loss  of  approximately  $16.50 
per  tree  or  $200  per  acre  has  been  caused  by  the  alfalfa.  The  alfalfa 
was  planted  in  the  middles  only  taking  up  actually  one-half  of  the 
ground  in  the  grove  as  shown  by  figure  29.  Crediting  the  alfalfa  at 
a  safe  estimated  production  and  at  the  ruling  prices  during  the  respec- 
tive years,  with  due  allowance  for  cost  of  production,  the  loss  due  to 
alfalfa  is  materially  reduced.  Even  then  the  clean  culture  plot  has 
been  the  most  profitable.  The  economic  side  of  the  question  will  vary 
from  year  to  year  with  the  prices  of  walnuts  and  alfalfa.    The  greatest 


72  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

interest  is  in  the  fact  that  apparently  the  alfalfa  as  an  intercrop  on 
one-half  of  the  land  was  a  detriment  to  walnut  trees  even  though  they 
are  planted  60  feet  apart.  The  cause  of  the  bad  effect  is  not  entirely 
clear.  An  abundance  of  irrigation  water  was  available  and  was  used 
more  freely  than  most  walnut  growers  could  afford  to  use  water. 
Studies  on  the  nitrate  content  of  soils  in  leguminous  cover  crops  com- 
pared with  clean  culture  lead  to  the  natural  belief  that  there  was  actual 
marked  reduction  of  nitrate  nitrogen  in  the  soil  of  the  alfalfa  plots. 

If  the  grower  has  only  a  small  plantation,  not  justifying  the 
maintenance  of  bean  farm  machinery,  and  has  not  a  ready  market  for 
a  vegetable  crop,  the  intercropping  with  a  precocious  fruit  tree  may 
be  desirable.  Peaches,  prunes,  and  apricots  are  among  the  fruit  trees 
most  commonly  used  in  such  schemes  of  intercropping.  In  many 
instances  observed  young  groves  interplanted  with  these  fruits  have 
made  less  growth  than  groves  interplanted  with  beans.  Nevertheless, 
in  many  cases,  the  fruit  trees  have  been  profitable  and  have  made  the 
orchard  self-supporting  while  the  nut  trees  were  growing,  thus  out- 
weighing, from  a  business  point  of  view,  the  temporary  check  of  the 
walnut  trees.  If  fruit  trees  are  used  as  an  intercrop,  the  farmer  must 
study  his  irrigation  problem  carefully  to  be  sure  the  walnut  trees  are 
not  robbed  of  their  needed  water  by  the  companion  crop.  General 
observations  indicate  that  the  apricot  tree  is  a  keener  competitor  of  the 
walnut  than  the  peach. 

There  are  several  serious  objections  to  interplanting  with  fruit 
trees.  If  cling-stone  canning  peaches  are  used  as  an  intercrop,  they  will 
require  late  summer  irrigation  to  fully  develop  their  crop,  at  which 
time  water  should  be  discontinued  upon  the  walnuts.  Some  of  the  latest 
and  most  profitable  varieties  should  be  watered  in  late  August  and 
September  in  order  to  bring  them  to  a  good  size  for  the  canning  factory. 
This  late  water  on  young  walnut  trees  is  very  liable  to  keep  the  twigs 
growing  so  late  that  they  will  be  severely  injured  by  the  early  fall 
frost.  Such  a  hazard  will  diminish  as  the  walnut  trees  come  into 
bearing  and  the  drain  of  crop  production  acts  as  a  regulator  in  slow- 
ing up  the  late  summer  growth,  thus  aiding  the  walnut  trees  to 
become  nearly  dormant  at  the  season  of  early  fall  frosts.  The  prune 
has  been  a  profitable  intercrop  with  walnuts,  but  here  the  discordant 
water  requirements  are  somewhat  reversed,  and  do  not  become  pro- 
nounced until  both  the  prunes  and  the  walnuts  are  in  bearing.  At  this 
stage  in  the  development  of  the  plantation,  when  it  is  possibly  from 
six  to  twelve  years  old,  and  before  it  seems  economically  sound  to  pull 
out  the  prunes,  the  late  summer  water  requirements  are  quite  opposite 
with  these  two  crops.    As  the  walnuts  mature  they  should  be  irrigated 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


73 


so  that  they  will  be  fully  developed  and  so  that  the  husks  will  crack 
open,  producing  a  clean-shelled,  plump -kerneled  nut ;  irrigation  of  the 
prunes  at  this  time  is  obectionable,  as.  it  lowers  their  sugar  content, 
and  it  is  generally  believed  it  will  also  cause  them  to  crack  open.  The 
result  may  thus  be  an  inferior  product,  liable  to  rejection  by  the  prune- 
selling  agencies. 

If  either  prunes  or  peaches  are  used  as  interplants  it  would  seem 
advisable  to  plant  the  walnuts  30'  x  60'  and  the  fruit  trees  in  solid 
rows  in  the  middle  of  the  interspaces.  These  temporary  trees  might  be 
planted  only  15  feet  apart  in  the  row,  which  would  give  enough  fruit 
trees  to  the  acre  to  make  it  worth  while,  and  yet  have  room  enough 
for  heavy  fruit  production  for  a  few  years.  This  arrangement  would 
permit  irrigating  only  a  part  of  the  land  thus  serving  the  water 
requirements  of  one  crop  without  affecting  its  associate  crop,  or  only 
slightly.  As  the  trees  grow  older,  the  roots  become  interwoven 
throughout  the  entire  soil  mass  and  gradually  the  advantages  of  this 
plan  lessen,  and  the  filler  trees  should  then  be  pulled  out. 

FERTILIZATION 

The  walnut  crop  makes  relatively  small  demands  upon  the  plant 
food  of  the  soil  in  comparison  to  other  fruit  crops.  This  may  be 
illustrated  by  Table  6. 

TABLE  6 

Plant  Food  Elements  Withdrawn  from  the  Soil  by  Various  Fruits, 

Expressed  in  Pounds  Per  Acre 


Fruit 

Amount  of  crop 

Nitrogen 

Phosphoric  acid 

Potash 

Oranges 

16,300  lbs. 

18,900  lbs. 

10,000  lbs. 

1,000  lbs. 

28.23  lbs. 
28.53  lbs. 
19.73  lbs. 
10.20  lbs. 

8.63  lbs. 

11.52  lbs. 

6.40  lbs. 

2.78  lbs. 

34.39  lbs. 

Lemons 

50.84  lbs. 

Apricots 

29  00  lbs. 

Walnuts 

1  50  lbs. 

Field  trials  with  various  kinds  and  amounts  of  fertilizers  applied 
to  several  walnut  groves  are  now  being"  carried  on  by  the  University 
of  California,  cooperating  with  the  Field  Department  of  the  California 
Walnut  Growers'  Association. 

Three  separate  trials  far  removed  from  each  other  on  different 
soil  types  have  continued  for  four,  five,  and  six  years,  respectively. 
The  small  difference  between  the  several  plots  thus  far  in  the  trial  are 
all  within  the  limits  of  error,  so  it  can  not  be  said  with  confidence  that 
the  fertilizers  have  been  in  the  least  profitable  or  even  beneficial. 


74  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Nitrogenous  fertilizers  have  apparently  increased  walnut  produc- 
tion on  some  types  of  soil  where  the  trial  of  farmers  have  been  observed. 
Their  use,  however,  should  be  considered  an  experiment,  and  only  a 
part  of  the  grove  should  be  so  treated  until  marked  results  are  obtained 
on  each  individual  property. 

PEUNING 

Like  many  of  the  fruit  trees,  such  as  apples  or  oranges,  the  pruning 
of  the  walnut  tree  has  evolved  into  several  similar  systems  of  pruning 
practice.  In  many  groves  pruning  consists  merely  in  the  cutting  off 
of  the  lower  limbs  which  interfere  with  cultural  practice. 

The  fact  that  few  walnuts  are  produced  in  the  centers  of  the  old 
trees  has  suggested  a  moderate  thinning  out  of  the  branches  from  year 
to  year  in  an  attempt  to  promote  production  more  uniformly  through- 
out the  trees.  Sunlight  is  necessary  for  the  production  and  mainten- 
ance of  fruit  spurs,  and  without  some  thinning  out  practically  all  of 
the  crop  is  produced  on  the  outside  twigs,  in  the  tops,  and  on  the  sides 
of  the  trees.  Some  of  the  most  successful  walnut  growers  have  adopted 
this  sort  of  pruning  as  an  annual  practice,  cutting  out  the  water 
sprouts  and  the  dead  wood,  thinning  out  the  thickest  growing  portion 
of  the  top,  and  reducing  the  number  of  crossed  limbs  and  weak  crotches. 
In  the  absence  of  any  results  of  systematic  field  trials  this  gradual 
and  annual  thinning-out  process  seems  the  most  advisable. 

This  system  of  pruning  should  not  include  the  heading  back  of 
fruiting  limbs,  as  such  a  practice  will  promote  a  rapid  growth  of  water 
shoots  near  the  ends  of  the  parts  which  remain,  defeating  the  very 
attempt  to  shorten  the  limbs.  If  the  limbs  are  too  long  and  top-heavy 
on  the  sides  of  the  trees,  they  should  be  cut  out  entirely,  or  cut  off  to 
lateral  limbs,  thus  restricting  in  a  measure  the  development  of  water 
shoots. 

In  all  pruning  operations  with  the  walnut,  care  should  be  taken  to 
cut  the  limbs  off  smoothly  with  a  saw  without  leaving  any  stub.  All 
wounds  of  two  inches  in  diameter  or  over  should  be  painted  with  a 
weatherproof  paint.  The  walnut  wood  decays  very  readily  and  there- 
fore any  large  cut  exposed  to  the  weather  for  a  short  period  will  surely 
become  infected  and  start  a  decay  which  will  eventually  spread  to  the 
center  of  the  tree.  The  framework  of  the  tree  thus  undermined  by 
so-called  heart  rot,  may  be  split  by  an  overload  of  nuts,  and  be 
partially  or  wholly  destroyed. 

Old  groves  in  which  the  trees  are  planted  very  close  together 
should  not  be  severely  pruned  to  prevent  the  branches  from  adjacent 
trees  interfering,  but  rather  a  number  of  the  trees  should  be  cut  out 


Bulletin  379]  WALNUT  CULTURE  IN   CALIFORNIA  75 

thus  thinning  out  the  grove  itself.  Figure  7  shows  a  close  grove  in 
which  the  lower  branches  have  been  periodically  cut  off,  finally  reduc- 
ing the  bearing  area  of  the  individual  trees  nearly  50  per  cent.  The 
production  in  such  groves  declines  prematurely  and  does  not  respond 
to  the  above  mentioned  pruning  system.  The  greatest  competition  in 
closely  planted  groves  is  in  the  root  systems  of  the  trees.  The  only 
remedy  is  to  remove  the  cause  by  taking  out  part  of  the  trees.  Figure  8 
shows  a  grove  which  was  thinned  out  by  removing  one-half  the  trees 
three  years  before  the  photograph  was  made.  The  old  stumps  are  still 
standing.  After  two  or  three  years  a  thinned  grove  may  be  expected 
to  return  to  its  production  before  the  removal  of  one -half  the  trees. 
After  this  interim  decided  gains  are  usually  made  over  the  former 
yields.  Even  though  the  trees  have  been  pruned  to  an  extremely 
upright  position  as  those  shown  in  figure  7,  after  the  grove  is  thinned 
out  lower  branches  will  grow  and  fill  out  to  a  more  oval  shaped  tree. 


INSECTS  AND  DISEASE  PESTS 

INSECTS 

The  only  insects  which  are  economically  troublesome  to  the  walnut 
are  the  codling  moth  (Cydia  pomonella),  the  aphis  (Chromaphis 
juglandicola) ,  and  the  red  spider  (Tetranychus  telarius)  ;  these  are 
mentioned  in  their  probable  order  of  importance. 

CODLING  MOTH  AND  APHIS 

These  two  insects  may  well  be  considered  at  the  same  time  for, 
where  they  both  occur,  control  methods  for  both  may  be  combined  in 
one  application. 

The  codling  moth  has  been  known  to  attack  walnuts  in  California 
since  1909,  when  it  was  first  observed  in  the  vicinity  of  Concord.* 
Since  its  first  appearance,  it  has  periodically  become  very  troublesome 
in  several  of  the  walnut  districts  of  southern  California. 

The  injury  caused  by  the  codling  moth  to  walnuts  is  similar  to 
that  seen  in  wormy  apples.  The  worm  eats  a  portion  of  the  kernel 
of  the  nut  and  renders  the  remainder  uneatable.  The  manner  of  attack 
and  entrance  of  the  worm  is  clearly  shown  in  figure  30. 

The  aphis  is  a  small  green  plant-louse  which  feeds  on  the  leaves 
and  succulent  growth  by  sucking  their  juices.  This  weakens  the  tree 
at  a  critical  stage,  causes  it  to  produce  less  crop,  lighter  and  smaller 


*  The  Codling  Moth  in  Walnuts.     H.  J.  Quayle,  Monthly  Bull.,  Dept.  Agr. 
California,  vol.  IX,  No.  3,  1920,  p.  64. 


76 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


nuts,  and  imperfect  shells.  The  foliage  of  a  tree  infested  with  this 
insect  becomes  covered  with  "honey  dew,"  a  sticky  secretion  which 
turns  black  upon  continued  exposure  to  the  air.  The  damage  caused 
by  the  aphis  varies  from  year  to  year.  In  some  sections  the  damage 
is  considerable  every  year,  in  others  only  in  exceptional  years.  The 
aphis  usually  disappears  with  the  approach  of  extremely  hot  weather. 


Fig.  30. —  (A)  Appearance  of  green  nut  after  worm  has  entered  on  open  side. 
(B)  Codling  moth  worm  usually  enters  green  nut  on  the  stem  end  as  here 
shown.  (C)  A  round  or  oval  opening  on  the  stem  end  of  the  cured  nut  is  almost 
sure  sign  that  the  worm  has  paid  his  respects  inside.  (D)  Section  of  walnut 
showing  the  codling  moth  worm. 


THE  CONTROL  OF  THE  CODLING  MOTH  ALONE 

Where  the  codling  moth  has  been  present  in  orchards  in  past  years 
and  the  amount  of  infestation  has  been  about  5  per  cent  or  greater,  the 
trees  should  be  dusted  or  sprayed  with  basic  arsenate  of  lead  between 
May  25  and  June  20,  and  a  second  spray  when  necessary,  during  the 
latter  half  of  July. 


Bulletin  379]  walnut  CULTURE  IN   CALIFORNIA  77 

1.  Dusting. — Five  or  six  pounds  of  dust*  to  the  tree  is  required 
for  trees  of  medium  size,  and  7  to  10  pounds  for  the  largest  trees. 
A  thorough  application  should  be  made  to  cover  all  the  nuts  and  all 
green  parts  of  the  tree. 

2.  Spraying. — Use  10  pounds  of  dry  basic  arsenate  of  lead  to  a 
200-gallon  spray  tank.  For  a  tree  of  average  size,  20  gallons  of  spray 
is  necessary,  or  1  pound  of  arsenate  of  lead.  For  the  largest  trees. 
30  gallons  of  spray  is  necessary,  or  V-/2  pounds  of  arsenate  of  lead  to 
a  tree.  All  nuts  on  the  trees  should  be  reached  by  the  spray.  It  is 
not  necessary  to  spray  the  trunk  and  branches  or  any  part  of  the  tree 
where  there  are  no  nuts,  unless  the  aphis  is  to  be  combatted  at  the 
same  time.  Spray  guns  should  be  used  in  order  to  reach  all  parts  of 
the  tree.  Spraying  will  insure  a  higher  degree  of  control  of  the  codling 
moth  than  dusting,  but  the  labor  expense  is  somewhat  more  and  it 
takes  longer  to  cover  a  given  acreage. 

APHIS  AND  CODLING  MOTH 

Where  past  experience  has  shown  that  the  walnut  aphis,  as  well  as 
the  codling  moth,  may  generally  be  present  in  large  numbers,  use  a 
dust  combination  containing  both  tobacco  and  arsenate  of  lead.  If 
spraying  is  to  be  practiced  instead  of  dusting  for  the  codling  moth, 
add  one  pint  of  ' '  black-leaf  forty ' '  to  each  200-gallon  tank  of  arsenate 
of  lead. 

APHIS  ALONE 

Where  there  is  no  codling  moth,  but  the  aphis  is  generally  present 
in  serious  numbers,  use  Nico-dust,  a  material  especially  prepared  for 
aphis  control.  From  2  to  6  pounds  of  this  material  is  necessary  for  a 
tree,  according  to  its  size.  The  best  general  time  for  such  treatment 
is  during  the  last  week  of  May  and  the  first  two  weeks  of  June.  During 
years  when  the  aphis  occurs  in  great  numbers,  it  may  be  advisable  to 
dust  a  second  time,  during  July  or  August. 

EED  SPIDER 

In  some  years  the  red  spider  does  much  damage  to  walnut  trees  in 
isolated  instances.  Injury  is  not  usually  apparent  until  the  middle  of 
the  summer,  when  the  leaves  turn  a  dull  but  decided  yellowish  hue. 
A  close  examination  at  this  time  will  show  the  presence  of  a  countless 
number  of  red  spiders,  barely  visible  to  the  naked  eye.  If  the  attack 
goes  on  unchecked,  the  leaves  will  drop  prematurely,  injuring  the 


*  Arsenate   of  lead  is   mixed  with   kaolin,   which   acts   as   a   diluent   and  a 
carrier.     Use  only  the  standard  commercial  brands. 


78  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

quality  of  the  current  crop  and  the  development  of  fruit  buds  for  the 
future  crop. 

The  most  practical  control  measure  is  dusting  with  dry  sulfur 
and  hydrated  lime,  three  parts  to  one,  respectively.  Spraying  during 
the  dormant  period  has  not  been  found  advisable  in  the  past,  because 
outbreaks  of  this  insect  have  not  been  frequent. 

WALNUT  BLIGHT  (Pseudomonas  juglandis) 
The  walnut  blight  is  by  far  the  most  destructive  disease  affecting 
the  walnut  crop.  The  prevalence  of  this  disease,  as  previously  stated, 
varies  considerably  from  year  to  year,  and  is  usually  much  worse  in 
the  foggy  coastal  districts  than  in  the  inland  valleys.  In  years  of  bad 
outbreaks,  the  blight  has  probably  destroyed  from  15  to  20  per  cent 
of  the  crop.  It  is  a  bacterial  disease  which  attacks  the  young  and 
tender  growth  and  spreads  to  the  more  mature  wood,  causing  the 
affected  areas  to  turn  black  and  die.  Under  favoring  conditions,  the 
disease  is  especially  destructive  to  the  nuts.  If  it  attacks  the  nuts 
early,  it  causes  them  to  turn  black  and  drop  off  when  one-eighth  to 
one-half  of  an  inch  in  diameter.  It  causes  mature  nuts  to  remain 
undeveloped,  and  may  make  full-grown  nuts  unmarketable,  except  as 
culls.  It  shows  on  the  nut  as  black  spots  most  prevalent  at  the  calyx 
end,  but  often  scattered  over  its  entire  surface. 

At  present  there  is  no  control  known  for  this  disease.  Attempts 
to  lessen  the  prevalence  of  the  blight  by  spraying  and  by  the  combina- 
tion of  spraying  and  pruning  off  the  old  blight  cankers  from  the  twig 
growths,  have  had  no  measurable  effect  upon  the  disease.* 

Some  of  the  varieties  now  being  propagated  are  more  resistant  to 
blight  than  the  average  seedling  tree.  At  present  the  greatest  likeli- 
hood of  relief  from  this  disease  lies  in  securing  resistant  or  immune 
varieties. 

MELAXUMA 

This  troublesome  disease  of  the  walnut  tree  has  occurred  in  isolated 
and  serious  outbreaks  in  Santa  Barbara,  Ventura,  Los  Angeles,  and 
Orange  counties.  Careful  studies  of  this  disease  were  made  and 
reported  in  1914  to  1915  by  Fawcett.*  The  nature  and  treatment  of 
the  trouble  may  best  be  summarized  by  quoting  from  the  above  author. 
(For  a  detailed  discussion  of  the  disease,  see  the  original  publication.) 

"Because  of  the  oozing  of  dark  watery  material  to  the  surface  of  the  affected 
areas,  this  disease  is  often  confused,  under  the  name   of  "black  sap,"  with 


*  An  Attempt  to  Control  Walnut  Blight.  H.  S.  Fawcett  and  L.  D.  Bachelor. 
Monthly  Bulletin,  California  Dept.  Agr.,  vol.  IX,  No.  5-6,  1920. 

t  Melaxuma  of  the  Walnut,  H.  S.  Fawcett,  Univ.  of  Calif.  Experiment  Station 
Bull.  No.  261,  1915. 


Bulletin    379]  WALNUT    CULTURE    IN    CALIFORNIA 


79 


Fig.  31. — One  of  the  first  evidences  of  Melaxuma.  The  black  sappy  ooze  has 
appeared  at  the  outer  edges  of  the  killed  bark  at  the  crotch  shown  by  arrow. 
The  bark  between  has  been  killed  and  pycnidia  of  Dothiorella  have  already 
appeared  on  the  surface  of  part  of  the  killed  bark.     (From  Fawcett,  Bulletin  261.) 


80 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


sunburn,  frost  injury,  injuries  to  the  bark  in  cultivation,  injury  from  the  decay 
of  wood  at  places  where  limbs  have  been  cut  off,  and  other  troubles  in  which  a 
"black  sap"  may  ooze  out  during  the  active  growing  period  of  the  tree.  It 
should  not  be  taken  for  granted,  therefore,  that,  because  a  black  ooze  is  seen 
on  the  trunk  or  larger  limbs  of  a  walnut  tree,  Melaxuma  is  necessarily  present. 


Fig.  32. — Walnut  trees  showing  the  location  of  Melaxuma  cankers  that  had 
been  cut  out  and  treated  with  Bordeaux  paste  the  year  previous.  Notice  the 
rows  of  holes  in  the  bark  of  one  tree,  made  by  sapsuckers.  These  probably 
served  in  this  case  as  a  partial  means  of  infection.    (Fawcett,  H.  S.,  Bulletin  261.) 


As  the  term  "black  sap"  has  already  been  used  by  R.  E.  Smith  to  designate 
a  result  of  sunburning,  this  name  was  not  considered  suitable  for  the  disease 
here  being  considered.  The  word  Melaxuma,  derived  from  two  Greek  words  mean- 
ing "black"  and  "juice"  was  therefore  adopted,  and  is  now  in  fairly  common 
use  to  designate  the  disease  herein  described. 


Bulletin    379]  WALNUT    CULTURE   IN    CALIFORNIA  81 


Description  of  the  Disease 

Melaxuma  shows  its  effect  most  strikingly  during  the  summer  after  the 
growth  is  well  started.  On  trees  severely  affected  at  this  time,  there  will  be 
seen  large  black  sunken  cankers  on  the  trunk  and  larger  limbs  and  often  a  sud- 
den wilting  of  the  smaller  limbs  and  twigs.  This  sudden  wilting  of  the  smaller 
limbs  is  so  different  from  the  effects  of  walnut  blight  or  bacteriosis,  that  it  is 
easily  distinguished  from  that  disease. 

The  most  common  location  for  Melaxuma  cankers  is  at  the  crotch  of  the  tree 
where  the  first  limbs  join  the  trunk.  The  first  evidence  of  the  disease  is  often  a 
black  area  on  the  otherwise  grayish  bark  which  looks  like  a  dab  of  tar,  as  shown 
by  Fig.  31.  This  is  due  to  the  staining  of  the  bark  by  a  black  watery  substance 
that  forms  under  it.  The  diseased  area  later  becomes  slightly  sunken,  shrinks, 
and  cracks.  The  "black  sap"  then  oozes  out  in  considerable  quantities  and 
stains  the  bark  as  it  runs  down  the  limb  or  trunk.  The  wood  underneath  is  dis- 
colored for  a  short  distance  and  this  discoloration  usually  extends  beyond  the 
margin  of  the  killed  bark.  The  diseased  areas,  as  a  rule,  do  not  extend  entirely 
around  a  limb  in  one  season,  but  affect  only  about  one-third  or  one-half  the 
circumference  of  the  bark.  Later  in  the  summer  or  fall  the  increase  in  the 
size  of  the  areas  is  slower  and  often  appears  to  stop.  One  part  of  the  margin  of 
the  canker  may  dry  out  and  begin  to  heal  over,  while  the  other  continues  to 
advance  or  remains  stationary  until  the  next  spring,  when  it  begins  to  advance 
rapidly  again.  A  few  cankers  on  large  limbs  of  vigorous  trees  may  even  heal 
over  without  treatment.  More  often  there  is  an  enlargement  from  year  to 
year  which  in  two  or  more  years  extends  entirely  around  a  large  limb,  causing 
it  to  wilt  and  die.  These  limbs  occasionally  wilt  suddenly,  the  dried-up  nuts 
and  leaves  remaining  attached  for  some  time.  When  cankers  occur  on  the 
trunks  they  generally  follow  some  injury  to  the  bark  made  by  a  plow,  cultivator 
or  other  instrument  used  in  the  cultivation  of  the  orchard. 

'  From  the  observations  and  results  of  experiments  so  far  made,  the  following 
tentative  treatment  is  suggested: 

Cut  out  the  cankers  that  ha^ve  not  gone  too  far  on  the  trunk  and  larger  limbs 
and  disinfect  the  wounds  thus  made.  (See  fig.  32.)  The  dead  and  discolored 
bark  should  be  cut  away,  getting  a  little  beyond  the  margin  of  dead  tissue. 
If  the  cankers  are  not  large  and  the  wood  underneath  has  not  been  stained 
deeply,  it  will  pay  to  dissect  all  the  discolored  wood  as  well  as  the  baj-k. 
Probably  one  of  the  best  disinfectants  to  apply  to  the  wound  is  the  Bordeaux 
paste,*  that  is  recommended  fof  lemon  gummosis.     If  the  canker  is  large  and 

*  Bordeaux  Paste.  The  formula  for  Bordeaux  paste  is  as  follows:  12  pounds 
of  bluestone  (copper  sulfate)  dissolved  in  8  gallons  of  water  in  a  wooden, 
earthen  or  glass  vessel;  and  24  pounds  of  quicklime  slaked  in  8  gallons  of  water. 
When  the  lime  is  cool,  stir  together  about  equal  parts  by  volume  of  each,  for 
making  enough  mixture  to  last  for  one  day  only.  The  bluestone  is  most  easily 
dissolved  by  suspending  it  in  a  sack  at  the  top  of  the  water.  If  the  bluestone  is 
pulverized  and  suspended  in  warm  water,  it  dissolves  rapidly.  Good  lime  that 
is  not  air-slaked  should  be  used,  and  after  slakjng  it  with  the  water,  it  should 
be  allowed  to  cool  before  being  used  in  making  pasje.  If  covered  to  avoid 
evaporation  each  ingredient  will  keep  indefinitely,  but  after  mixing,  the  paste 
slowly  deteriorates.  Where  it  is  being  used  over  a  number  of  days  or  weeks, 
just  enough  of  the  wet  slaked  lime  and  the  bluestone  solution  should  be  mixed 
to  make  paste  enough  to  last  for  one  day,  leaving  the  remainder  unmixed  in 
separate  vessels.     It  may  be^applied  with  large  brushes,  as  in  whitewash. 


82  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

has  been  in  the  tree  a  long  time,  the  wood  may  be  stained  so  deeply  as  to  render 
the  work  of  cutting  out  all  discolored  wood  too  expensive.  If  the  canker  has 
practically  girdled  the  limb,  the  limb  had  better  be  cut  out. ' ' 

WINTEE  INJUEY  OE  DIE-BACK 

This  trouble  of  the  walnut  tree  has  been  made  the  subject  of 
considerable  study  by  Smith*  and  Batchelor  and  Reed.f  Perhaps  it 
can  be  summarized  best  at  this  time  by  quoting  from  the  last  mentioned 
authors. 

Winter  injury  or  die-back  of  walnuts  is  characterized  by  a  death 
of  the  tops  of  the  trees.  Such  injury  is  usually  first  noticeable  during 
the  early  spring. 

The  most  common  causes  of  the  winter  injury  or  die-back  are  given 
below. 

1.  Early  autumn  frosts  kill  the  immature,  growing  shoots.  Young 
walnut  trees  are  more  subject  to  injury  from  this  cause  than  older 
trees,  because  they  are  usually  later  in  maturing  their  new  wood.  Such 
frosts  cause  the  foliage  to  drop  prematurely  and  injure  the  growing 
tips  of  the  twigs.  The  denuded  twigs  are  subject  to  further  injury 
from  subsequent  fall  and  winter  sunburn.  The  presence  or  extent  of 
this  injury  is  usually  overlooked  until  the  following  spring. 

To  reduce  the  danger  from  autumn  frosts  it  is  advisable  to  withhold 
the  late  summer  irrigation  in  order  to  promote  the  early  maturity  of 
the  trees. 

2.  Winter  drought  causes  die-back  in  either  young  or  bearing 
walnut  groves.  Trees  suffering  from  this  condition  fail  to  make  new 
growth  in  the  spring,  except  from  the  trunks  or  main  limbs.  The  new 
growth  on  such  trees  has  lost  so  much  water  during  the  winter  that 
the  buds  are  unable  to  develop  in  spring.  The  cause  of  the  die-back 
has  been  found  to  be  due  to  an  extremely  low  moisture  content  of  the 
soil  during  a  large  part  of  the  winter.  Under  such  conditions,  the 
water  lost  from  the  young  shoots  during  the  winter  cannot  be 
replenished  by  the  root  system,  and  the  shoots  die  from  desiccation. 

Fall  and  winter  irrigation  of  the  walnut  groves  has  been  found  to 
eliminate  the  injury  from  winter  drought.  The  amount  of  irrigation 
water  to  be  applied  will  depend  upon  the  type  of  soil,  the  amount  of 
soil  moisture  present  at  the  end  of  the  harvest  season,  and  the  rainfall 
which  may  be  expected  later  in  the  season. 


*  E.  E.  Smith,  Walnut  Culture  in  California,  Univ.  Calif.  Experiment  Station 
Bull.  No.  231,  1912,  p.  372. 

t  L.  D.  Batchelor  and  H.  S,  Eeed,  Winter  Injury  or  Die-back  of  the  Walnut, 
Univ.  of  Calif.  Experiment  Station  Cir.  No.  216,  1919. 


Bulletin  379] 


WALNUT    CULTURE    IN    CALIFORNIA 


83 


Figure  33  shows  a  walnut  tree  thus  affected  and  afterward  cured 
by  winter  irrigation. 

The  late  fall  or  early  winter  irrigation  of  the  walnut  groves, 
especially  in  the  inland  valleys,  may  be  looked  upon  as  an  insurance 
policy  against  this  form  of  die-back.  To  be  effective,  it  should  be 
applied  in  December  or  January.  In  some  years  subsequent  rains  will 
show  that  the  die-back  insurance  policy  was.  unnecessary.  The  same 
apparent  objection,  however,  applies  to  fire  insurance  on  the  barn  or 
house  during  the  years  that  there  are  no  fires  on  the  property. 


Fig.  33. — Showing  the  effect  of  winter  drought  and  recovery  of  the 
tree  two  years  later  after  irrigation. 


3.  A  high  water-table  may  be  a  contributing  factor  in  die-back. 
A  permanently  high  water-table  may  cause  young  trees  to  prolong 
their  growing  season,  with  the  result  that  they  are  killed  by  frosts. 
The  sudden  rise  of  a  fluctuating  water-table  may  kill  a  large  part  of 
the  root  system  and  thus  cause  a  reduction  of  the  moisture  conducted 
to  the  top  of  the  tree.  This  in  turn  will  produce  a  typical  die-back  in 
the  tops  of  mature  trees. 

4.  Alkali  soils  containing  such  a  high  salt  content  as  to  injure  the 
roots  systems  of  walnut  trees  also  cause  the  tops  of  the  trees  to  die 
back  in  response  to  the  root  injury.     In  the  initial  stages  of  alkali 


84  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

injury  the  leaves  turn  brown  at  the  margin  and  fall  prematurely.  The 
denuded  shoots  sometimes  put  out  a  new  set  of  leaves  in  the  fall.  The 
top  of  the  tree  gradually  dies  back  until  the  entire  tree  is  lost. 

If  the  injury  is  being  caused  by  irrigation  water  containing  too 
much  alkali,  obviously  the  use  of  such  water  should  be  discontinued. 
Trees  which  have  been  severely  injured  by  this  cause  have  been  known 
to  recover  to  a  marked  degree  after  the  use  of  suitable  irrigation  water 
for  several  years. 


HARVESTING,  CURING,  PACKING,  AND  COST  OF 
PRODUCTION 

HARVESTING 

During  favorable  seasons  and  in  well-cared-for  groves  the  husks 
of  the  nuts  crack  open  and  adhere  temporarily  to  the  twigs,  while  the 
nuts  drop  clean-shelled  to  the  ground.  This  natural  dropping  of  the 
great  bulk  of  the  nuts  occurs  with  most. varieties  between  September  1 
and  November  7.  There, are  usually  enough  nuts  on  the  ground  to 
justify  the  first  picking  by  the  second  week  in  September.  The  natural 
falling  of  the  nuts  is  hastened  during  the  harvesting  process  by  shaking 
the  trees  with  long  poles  having  hooks  attached  to  the  ends. 

During  the  harvest  period  the  nuts  are  picked  up  at  least  three 
or  four  times  before  the  total  crop  has  matured  and  dropped.  It  is 
not  considered  good  practice  to  allow  the  nuts  to  remain  for  a  long 
time  on  the  ground.  Nuts  thus  neglected  are  subject  to  the  work  of 
ants  in  the  kernels,  and  may  be  rained  on  by  the  autumn  showers. 
The  effect  of  rain  upon  the  nuts  may  be  entirely  superficial  if  they 
are  picked  up  promptly  after  they  are  dry  again.  At  the  best,  how- 
ever, they  will  be  dirty  and  more  or  less  stained,  causing  extra  work 
of  washing  in  preparing  them  for  the  .packing  house,  and  making 
proper  bleaching  more  difficult.  Nuts  long  neglected  on  the  ground 
after  a  rain  become  moldy  and  stained  on  the  outside  of  the  shells, 
especially  those  which  have  a  portion  of  the  husk  adhering  to  them. 
If  the  stain  of  the  mold  is  pronounced  it  will  be  impossible  to  bleach 
it,  and  the  nut  must  be  graded  as  a  cull.  If  further  neglected  the  mold 
from  the  outside  of  the  nut  and  the  inside  of  the  attached  husk  may 
spread  to  the  kernel,  through  the  base  of  the  nut,  and  thus  lower  its 
value  even  as  a  cull  nut. 

A  certain  percentage  of  the  nuts  will  drop  with  the  husks  adhering 
to  them.  These  are  commonly  known  as  I '  stick-tights, ' '  and  are  likely 
to  be  inferior  to  the  clean  shelled  nuts  in  their  plumpness  and  in  the 


Bulletin  379]  WALNUT  CULTURE  IN   CALIFORNIA  85 

appearance  of  the  kernels.  There  is  usually  a  high  percentage  of 
blanks,  shriveled,  moldy,  and  dark-colored  kernels  among  the  stick- 
tight  nuts.  The  percentage  of  stick-tights  is  greater  during  seasons  of 
abnormally  high  temperatures,  when  the  nuts  are  sunburned  and  when 
the  husks  are  affected  with  blight.  Again,  trees  which  suffer  from 
drought  during  the  latter  part  of  the  growing  period,  or  which  are 
subject  to  the  attacks  of  the  aphis,  red  spider,  or  for  any  other  reason 
lose  their  leaves  prematurely,  produce  a  high  percentage  of  stick-tights 
and  inferior  nuts. 

A  small  per  centage  of  nuts  may  mold  after  the  husks  crack  and 
before  the  nuts  drop  from  the  trees.  "Stick-tight"  nuts  are  more 
likely  to  be  moldy  at  the  time  they  drop  from  the  trees  than  those 
which  drop  from  the  trees  free  from  their  husks.  The  percentage  of 
moldy  nuts  will  increase  if  "stick-tights"  are  left  on  the  ground  for 
a  week  or  ten  days.  Such  "stick-tight"  nuts  should  be  harvested 
with  dispatch  so  they  will  not  become  so  moldy  as  to  be  classed  as  culls.* 

WASHING  AND  CUEING 

After  being  picked  up  and  sacked,  the  nuts  are  ready  for  curing. 
If  most  of  them  are  dirty  from  lying  on  the  ground  during  showers 
it  is  necessary  to  wash  them.  This  washing  is  done  in  large  cylin- 
drical drums  made  of  coarse  wire  netting,  in  which  the  nuts  are 
slowly  revolved  under  a  stream  of  water,  grinding  against  each  other 
and  against  the  wires  which  form  the  sides  of  the  drum.  They  are 
thus  cleansed. of  all  loose  foreign  material.  This  washing  process  will 
not  remove  the  stains  on  the  shells  due  to  sunburned  and  blighted  husks 
sticking  to  the  nuts.  If.  nuts  are  to  be  washed  by  this  process  they 
should  pass  through  the  drum  in  a  continuous  stream  so  that  any 
individual  nut  will  not  be  in  the  drum  more  than  2  or  3  minutes.  The 
prolonged  rotation  of  a  lot  of  nuts  in  such  a  device,  cracks  them, 
loosens  and  bruises  the  meats,  and  will  soon  make  culls  of  the  whole 
lot.    There  is  no  need  for  washing  the  clean  nuts. 

The  curing  is  accomplished  by  spreading  the  nuts  out  in  shallow 
trays  with  bottoms  made  of  slats  spaced  about  one-half  an  inch  apart. 
The  nuts  should  not  be  left  exposed  to  the  sun  during  the  entire  day 
if  the  weather  is  especially  clear  and  hot,  for  the  drying  will  be  so 
rapid  that  many  of  the  nuts  will  crack  open.  If  the  trays  are  spread 
out  in  the  morning  and  the  nuts  thoroughly  stirred  several  times  the 
trays  may  be  piled  up  when  the  nuts  are  warm.    The  slow  drying  whicr 


.  *  For  a  detailed  discussion  of  the  effect  of  harvesting  methods  on  the  pre- 
valence of  moldy  nuts  see  Methods  of  Harvesting  and  Irrigation  in  Eelation 
to  Moldy  Walnuts,  by  L.  D.  Batchelor,  Univ.  of  Calif.  Bull.  No.  367,  1923. 


86 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


goes  on  while  the  trays  are  in  piles  of  course  prolongs  the  process,  but 
prevents  the  splitting  of  poorly  sealed  nuts.  Figure  34  shows  a  yard 
full  of  trays  ready  to  be  piled  up  after  being  thoroughly  warmed  up 
by  the  morning  sun.  The  trays  should  be  so  piled  as  to  allow  ventila- 
tion between  them.  At  night  the  nuts  should  be  protected  against 
exposure  to  fog  or  dew.  The  alternate  loss  and  gain  in  moisture  when 
the  nuts  are  exposed  to  fog  at  night  and  sunshine  in  the  day  may  be 
the  cause  of  splitting.  Well  cured  nuts  should  not  show  more  than 
2  or  3  per  cent  split  nuts ;  if  they  are  exposed  to  the  fog  over  40  per  cent 
may  split. 


Fig.  34. — Drying  walnuts  by  means  of  spreading  them  on  trays 
placed  on  low  racks. 


If  the  walnut  grove  is  large  enough,  it  will  save  expense  to  use  a 
' '  drying  house ' '  for  the  curing  process.  Such  ' '  dry  houses ' '  are  built 
with  outer  walls  of  lath  to  give  good  ventilation.  The  drying  bins  are 
arranged  one  above  another  in  a  single  column.  The  nuts  are  carried 
away  from  the  washing  machine  by  a  belt  and  thence  elevated  to  the 
upper  bin.  The  nuts  are  dumped  mechanically  from  one  bin  to  the 
bin  below  it.  Thus  they  work  through  the  whole  series  of  bins  from 
the  top  down.  There  may  be  seven  or  eight  of  these  bins  and  by 
dumping  them  each  once  a  day,  the  nuts  will  be  passed  through  the 
house  in  seven  or  eight  days.  As  they  leave  the  bins  after  this 
period,  they  are  usually  sufficiently  cured  to  be  ready  to  go  to  the 
packing  house,  to  be  bleached,  graded,  and  bagged  for  shipment.  If 
the  weather  is  warm  and  the  atmosphere  very  dry,  the  nuts  may  pass 


Bulletin  379]  WALNUT   CULTURE  IN   CALIFORNIA  87 

through  the  drying  house  in  four  or  five  days.     Much  will  depend 
also  upon  the  condition  of  the  nuts  when  they  go  into  the  house,  a 
well  as  upon  the  exposure  of  the  house  to  breezes,  sunshine,  etc. 

As  the  nuts  pass  along  a  belt  when  they  come  from  the  drying  bins 
or  as  they  are  sacked  from  the  trays,  they  should  be  carefully  sorted 
to  pick  out  the  culls.  Nuts  are  thrown  into  this  cull  class  if  they  arc 
cracked,  perforated,  badly  stained  by  blight  or  sunburn,  wormy,  or  if  a 
part  of  the  shuck  is  adhering  to  the  shell.  The  good  nuts  are  now  ready 
for  the  packing  house,  while  the  culls  are  kept  separate  to  go  finally  to 
the  cracking  plant.  The  prime  advantage  in  such  an  unheated  drying 
house  is  the  saving  in  labor.  The  overhead  costs  for  curing  in  such  a 
house  due  to  its  expensive  cost  of  construction,  largely  offset  the  labor 
saving  so  far  as  the  economic  side  of  the  problem  is  concerned.  A 
frequent  shortage  of  labor  during  the  harvest  season  may  make  the 
use  of  a  drying  house  of  especial  value  in  the  outlying  country  districts. 

All  the  advantages  of  a  drying  house  such  as  described  above  may 
be  obtained  in  a  dehydrater  with  the  added  advantage  of  a  great 
reduction  in  the  curing  time.  The  advantages  of  dehydration  may  be 
obtained  at  no  greater  cost  than  drying  in  the  sun.  These  advantages 
may  be  briefly  stated  as  follows  :* 


ADVANTAGES  OF  DEHYDRATION 

Careful  observation  of  the  operation  of  all  types  of  walnut  dehy- 
drater s  during  the  past  two  seasons  has  led  to  the  following  conclusions 
regarding  the  salient  advantages  of  dehydration  of  walnuts  as  com- 
pared with  sun-drying. 

1.  Weather  Protection. — Dehydraters  may  be  operated  constantly 
and  thus  continue  the  drying  process  day  and  night  in  all  kinds  of 
weather.  Splitting  of  nuts  by  dry,  hot  winds  or  delayed  drying 
because  of  fogs  or  ra^ns  are  both  avoided,  with  consequent  elimination 
of  worry,  extra  labor,  depreciation  in  the  quality  of  the  nuts,  and  a 
general  money  loss  to  the  grower  during  unfavorable  drying  weather. 

2.  Theft  Protection. — All  harvested  nuts  are  protected  from  theft 
by  being  dried  and  stored  within  the  dehydration  building  uri*til 
delivered  to  the,  packing  house. 

3.  Drying  Efficiency. — Accurate  control  of  the  drying  air  results 
in  the  most  uniform,  thorough  and  rapid  drying  consistent  with  the 
best  quality. 


*  For  a  more  detailed  account  of  dehydration  see  Sun  Drying  and  Dehydra- 
tion of  Walnuts,  by  Batchelor  and  Christie,  Univ.  of  Calif.  Bull.  No.  376,  1924. 


00  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

4.  Labor  Efficiency. — Dehydraters  require  considerably  less  labor 
than  sun-drying  or  the  use  of  a  dry-house.  This  is  an  important  con- 
sideration in  the  fall  of  the  year  when  the  harvesting  of  other  crops 
is  making  demands  on  the  labor  supply  in  competition  with  walnuts. 

5.  Packing  House  Efficiency. — Dehydraters  afford  a  steady  supply 
of  cured  nuts,  enabling  packing  houses  to  operate  more  nearly  at  their 
maximum  efficiency  throughout  the  season. 

PACKING 

Although  the  packing  and  selling  of  the  walnut  crop  in  California 
is  quite  distinct  from  its  production,  it  is  nevertheless  desirable  for 
each  grower  to  know  how  his  nuts  are  to  be  graded  and  packed,  in  order 
that  he  may  realize  more  fully  the  importance  of  delivering  high-grade 
nuts  to  the  packing  house.  About  85  per  cent  of  the  walnut  crop 
of  California  is  packed  and  sold  through  the  local  houses  which  are 
affiliated  with  the  California  Walnut  Growers'  Association.  It  may 
therefore  be  useful  to  relate  briefly  the  processes  followed  in  these 
houses  in  preparing  the  crop  for  shipment. 

Eliminating  Imperfect  Nuts.— After  being  properly  cured,  the  nutf 
are  delivered  to  the  local  packing  house  by  the  grower.  They  are  first 
run  over  a  rough  screen  which  frees  them  from  dirt  and  all  foreign 
matter.  From  this  screen  the  nuts  pass  through  a  suction  machine, 
a  device  which  lifts  the  blank  or  imperfectly  filled  nuts  over  a  trap 
and  allows  the  full-meated  nuts  to  pass  through.  From  the  suction 
machine  the  full-meated  nuts  pass  on  to  an  endless  belt  where  they  are 
culled  to  remove  the  stained,  cracked,  ill-shapen,  and  unsightly  nuts. 

Bleaching. — The  next  treatment  is  that  of  bleaching.  The  nut? 
are  given  a  bath  of  from  one  to  two  minutes'  duration  by  passing  them 
through  large  drums  partially  filled  with  a  liquid  bleaching  solution, 
for  the  purpose  of  removing  dirt  and  stain  and  brightening  the 
appearance  of  the  shells. 

From  the  bleaching  drums  they  are  passed  over  a  shaker  which  frees 
them  from  accumulated  drops  of  the  solution.  They  are  then  elevated 
and  graded  for  size. 

Grading. — For  this  process  large  cylindrical  galvanized-iron  graders 
are  used.  Each  grader  is  10  feet  long  and  40  inches  in  diameter,  set  on 
a  6-inch  pitch  and  has  a  capacity  to  properly  grade  one  and  one-half 
tons  of  walnuts  an  hour.  A  grader  used  for  the  Santa  Barbara  soft- 
shell  seedlings  contains  somewhat  over  8000  openings  i%2  incn  square 
and  the  walnuts  which  pass  over  the  grader  without  falling  through 
these  openings  are  known  as  No.  1  size.  Those  which  do  fall  through 
comprise  the  ' '  Baby  Soft-Shell  size. ' ' 


Bulletin    379]  WALNUT    CULTURE   IN    CALIFORNIA  89 

Graders  to  be  used  for  budded  nuts  contain  somewhat  less  than 
8000  openings  1%6  inch  square,  and  the  nuts  passing  over  constitute 
the  Fancy  Grade  and  those  falling  through  the  Standard  Grade. 

A  commercially  graded  pound  of  No.  1  nuts  will  comprise  about 
42  to  45  nuts,  while  the  Baby  Soft-Shell  grade  requires  nearly  80  nuts 
to  make  a  pound.  With  the  latter  grade  requiring  nearly  twice  as 
many  nuts  to  make  a  pound,  often  selling  at  20  per  cent  less,  it  is  clear 
that  a  grower  should  use  every  cultural  and  irrigation  precaution  to 
prevent  the  production  of  small  nuts. 

From  the  grader  the  walnuts  again  pass  to  a  grading-belt,  where 
those  that  have  not  been  bleached  properly,  or  have  been  broken  by  the 
bleaching  and  grading  operations,  are  removed.  The  perfect  nuts 
then  go  into  the  drying-bins,  and  after  a  period  of  from  24  to  48  hours 
are  packed,  100  pounds  net  weight,  in  burlap  bags,  ready  for  shipment. 

STAND AED  FOR  FIRST  QUALITY  NUTS 

A  nut  is  considered  satisfactory  if  its  size  is  in  accordance  with  the 
specifications  of  the  No.  1  or  the  Baby  Soft-Shell  grade; (if  its  kerne" 
is  plump  and  sound,  not  too  dark  in  color ;  if  it  is  not  wormy,  moldy, 
or  rancid ;  and  if  one  portion  of  the  shell  has  not  split  and  fallen  away 
from  the  other.  The  shell  must  have  no  material  outward  blemish  and 
must  show  a  clean  bright  color.  ^ 

In  most  seasons  the  California  Walnut  Growers'  Association 
guarantees  that  at  least  90  per  cent  of  the  nuts  in  each  sack  sold  by 
them  comply  with  the  above  standard. 

SELLING  THE  CROP 

As  already  noted,  about  85  per  cent  of  the  walnut  crop  is  sold  by 
the  California  Walnut  Growers'  Association.  This  central  organiza- 
tion is  a  non-capital,  non-profit,  cooperative  association,  composed  of 
more  than  forty  local  packing  associations.  The  board  of  directors  is 
composed  of  a  member  from  each  local  packing  association.  The  local 
associations  are  also  organized  on  cooperative  principles.  The  local 
plants  grade  and  pack  the  nuts  of  their  members  in  accordance  with 
the  standard  agreed  upon  by  the  board  of  directors  of  the  central 
association.  The  central  association,  by  a  rigid  inspection,  maintains 
the  standards  set  and  performs  its  prime  function  of  selling  the  crop. 
The  walnut  growers  affiliated  in  this  way  receive  the  actual  selling 
price  of  their  nuts  minus  the  bare  cost  of  grading,  packing,  and  selling. 

The  walnut  growers  who  do  not  belong  to  the  cooperative  associa- 
tions usually  sell  their  crops  to  the  various  independent  packing  firms, 
who  later  grade  and  pack  the  nuts  according  to  their  own  standards. 


90 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE   7 

Estimate  of  the  Average  Returns  which  May  be  Expected  from 

California  Walnut  Groves 


Average 
yield  of 
the  State 

Safe 

estimate 

for  business 

purposes 

Possible 
but  extra- 
ordinary 

Yield  per  acre 

800  lbs. 

1200  lbs. 

2000  lbs. 

Value  of  property  per  acre 

$700 

$1000 

$1700 

Gross  income  per  acref  

$164.80 
79.27 
20.48 
19.92 

$247.20 
79.27 
30.72 
29.88 

$412.00 

Annual  cost  of  operation  per  acre* 

79.27 

Harvesting  and  curing  cost  (%. 0256c  per  lb.)* 

Annual  packing  and  marketing  (%. 0249c  per  lb.)*.. 

51.20 
49.80 

Total  charge  per  acre  against  crop  J   . 

119.67 

139.87 

180.27 

Net  income  per  acre§ 

45.13 

107.33 

231.73 

Interest  at  6%  on  valuation^ 

42.00 

60.00 

102.00 

Profit  per  acre|| 

3.13 

47.33 

129.73 

(above  what  might  have  been  realized  with  the 
capital  invested  in  safe  securities,  and  the  owner 
working  for  wages.) 

Interest  which  is  returned  to  capital  for  the  risk  of 
doing  business  over  and  above  the  return  which 
comes  from  safe  bonds  or  other  securities 

0.4% 

4.7% 

7.6% 

t  Based  on  the  average  selling  price  per  pound  of  No.  1  seedling  nuts  (20.6 
cents)  for  12  years,  1912-23  which  is  the  total  period  of  operation  of  the  Cali- 
fornia Walnut  Growers'  Association,  the  discount  on  No.  2  nuts  and  culls  might 
reduce  the  average  returns  1  cent  per  pound.  All  nuts  would  not  be  No.  1  grade, 
therefore,  the  above  returns  are  slightly  exaggerated.  Although  the  tonnage 
of  seedling  nuts  is  still  by  far  more  important  than  all  other  varieties  combined, 
the  planting  of  seedling  groves  has  been  practically  abandoned  for  the  past  10 
years.  The  returns  from  the  Placentia  variety  have  usually  been  3  cents  a 
pound  more  than  seedlings,  while  the  several  fancy  varieties  such  as  Eureka, 
Franquette  and  Payne  have  averaged  from  3  to  6  cents  a  pound  more  than 
seedlings. 

*  From  Brief  of  Facts  Relating  to  the  American  Walnut  Industry,  page  23, 
1921. 

t  Includes  irrigation,  pruning,  cultivation,  depreciation,  and  insurance,  reason- 
able charge  for  owners  supervision,  and  taxes. 

§  Gross  returns  minus  all  expenses. 

H  The  rate  of  6  per  cent  is  used  here  to  charge  against  the  grove  as  the 
return  which  might  have  been  made  had  the  owner  invested  in  securities  rather 
than  a  walnut  grove. 

||  The  gross  return  minus  all  expenses  and  6  per  cent  of  the  valuation  of  the 
property. 


BULLETIN    379 J  WALNUT    CULTURE   IN    CALIFORNIA  91 


INCOME  TO  BE  DERIVED  FROM  WALNUTS 

In  order  to  assist  the  beginner  in  considering  the  probable  profits 
to  be  derived  from  growing  walnuts  table  No.  7  has  been  prepared. 

The  average  yield  for  the  state  varies  from  year  to  year.  Numerous 
statistical  publications  indicate  that  a  ten-year  average  yield  for 
bearing  orchards  is  approximately  800  pounds  per  acre.  This  figure 
has  been  used  as  a  conservative  estimate  for  business  purposes  in  the 
calculations.  This  yield  will,  no  doubt,  seem  rather  low  to  many 
experienced  growers.  It  should  be  borne  in  mind,  however,  that  thG 
average  production  for  a  period  of  ten  years  of  many  good  groves  is 
considerably  lower  than  the  occasional  yield  of  the  same  groves.  Blight, 
frost,  heat,  and  other  natural  agencies  make  rather  frequent  inroads 
in  the  prospective  crops.  Groves  which  have  averaged  1500  pounds 
per  acre  for  a  period  of  ten  years  are  rare,  and  those  which  have 
averaged  2000  pounds  are  extraordinary. 

Hunt*  writes  that,  "A  competent  farmer  should  expect,  and  no  one 
should  undertake  to  farm  unless  he  may  reasonably  expect  to  produce 
50  per  cent  more  than  the  average.  On  this  basis,  the  gross  income  per 
annum  may  be  estimated  at  25  per  cent  of  the  capital  invested.  This 
does  not  mean  that  every  legitimate  farm  enterprise  will,  or  should 
bring  in  just  25  per  cent  of  the  capital  invested.  This  statement  i: 
only  meant  to  give  one  a  'yard  stick'  with  which  to  measure  any 
definite  farming  enterprise. 


•"Suggestions  to  the  Settler  in  California."     T.  F.  Hunt,  Univ.  of  Calif. 
Experiment  Station  Circ.  No.  210,  1919,  p.  4. 


STATION  PUBLICATIONS  AVAILABLE  FOR  FREE  DISTRIBUTION 


BULLETINS 
No.  No. 

253.   Irrigation   and   Soil   Conditions   in  the  346. 

Sierra  Nevada  Foothills,  California.  347. 

261.  Melaxuma    of    the    Walnut,     "Juglans 

regia."  348. 

262.  Citrus   Diseases   of   Florida   and  Cuba  349. 

Compared  with  Those  of  California. 

263.  Size  Grades  for  Ripe  Olives.  350. 
268.  Growing  and  Grafting  Olive  Seedlings.  351. 
273.   Preliminary  Report  on  Kearney  Vine-  352. 

yard  Experimental  Drain. 

275.  The  Cultivation  of  Belladonna  in  Cali-  353. 

fornia.  354. 

276.  The  Pomegranate.  357. 

277.  Sudan  Grass 

2  78.   Grain   Sorghums. 

279.  Irrigation  of  Rice  in  California.  358. 

280.  Irrigation  of  Alfalfa  in  the  Sacramento 

Valley.  359. 

283.  The  Olive  Insects  of  California.  360. 

285.  The  Milk  Goat  in  California. 

286.  Commercial  Fertilizers.  361. 

287.  Vinegar  from  Waste  Fruits. 

294.   Bean  Culture  in  California.  362. 

298.   Seedless  Raisin  Grapes.  363. 

304.  A   Study  of  the  Effects  of  Freezes  on 

Citrus   in   California.  364. 

310.  Plum  Pollination. 

312.  Mariout  Barley.  366. 

313.  Pruning  Young  Deciduous  Fruit  Trees. 

317.   Selections  of   Stocks   in   Citrus  Propa-  367. 

gation. 
319.   Caprifigs  and  Caprification.  368. 

321.   Commercial  Production  of  Grape  Syrup. 

324.  Storage  of  Perishable  Fruit  at  Freezing  369. 

Temperatures.  370. 

325.  Rice  Irrigation  Measurements  and  Ex-  371. 

periments     in      Sacramento     Valley, 
1914-1919.  372. 

328.   Prune  Growing  in  California. 

331.   Phylloxera-Resistant  Stocks.  373. 

334.  Preliminary  Volume  Tables  for  Second-  374. 

Growth  Redwood. 

335.  Cocoanut   Meal   as    a    Feed   for   Dairy 

Cows  and  Other  Livestock.  375. 

336.  The    Preparation   of   Nicotine   Dust   as 

an  Insecticide.  376. 

339.  The  Relative  Cost  of  Making  Logs  from 

Small  and  Large  Timber.  377. 

340.  Control  of  the  Pocket  Gopher  in  Cali-  378. 

fornia. 

343.  Cheese  Pests  and  Their  Control. 

344.  Cold  Storage  as  an  Aid  to  the  Market- 

ing of  Plums. 


Almond  Pollination. 

The  Control  of  Red  Spiders  in  Decidu- 
ous Orchards. 

Pruning  Young  Olive  Trees. 

A  Study  of  Sidedraft  and  Tractor 
Hitches. 

Agriculture  in  Cut-over  Redwood  Lands. 

California  State  Dairy  Cow  Competition. 

Further  Experiments  in  Plum  Pollina 
tion. 

Bovine  Infectious  Abortion. 

Results  of  Rice  Experiments  in   1922. 

A  Self-mixing  Dusting  Machine  for 
Applying  Dry  Insecticides  and 
Fungicides. 

Black  Measles,  Water  Berries,  and 
Related  Vine  Troubles. 

Fruit  Beverage  Investigations. 

Gum  Diseases  of  Citrus  Trees  in  Cali- 
fornia. 

Preliminary  Yield  Tables  for  Second 
Growth  Redwood. 

Dust  and  the  Tractor  Engine. 

The  Pruning  of  Citrus  Trees  in  Cali- 
fornia. 

Fungicidal  Dusts  for  the  Control  of 
Bunt. 

Turkish  Tobacco  Culture,  Curing  and 
Marketing. 

Methods  of  Harvesting  and  Irrigation 
in  Relation  to  Mouldy  Walnuts. 

Bacterial  Decomposition  of  Olives  dur- 
ing Pickling. 

Comparison  of  Woods  for  Butter  Boxes. 

Browning  of  Yellow  Newtown  Apples. 

The  Relative  Cost  of  Yarding  Small 
and  Large  Timber. 

The  Cost  of  Producing  Market  Milk  and 
Butterfat  on  246  California  Dairies. 

Pear  Pollination. 

A  Survey  of  Orchard  Practices  in  the 
Citrus  Industry  of  Southern  Cali- 
fornia. 

Results  of  Rice  Experiments  at  Cor- 
tena,    1923. 

Sun-Drying  and  Dehydration  of  Wal- 
nuts. 

The  Cold  Storage  of  Pears. 

Studies  on  the  Nutritional  Disease  of 
Poultry  Caused  by  Vitamin  A  De- 
ficiency. ' 


No. 
70.   Observations    on    the    Status    of    Corn 

Growing  in  California. 
87.  Alfalfa, 
111.  The  Use  of  Lime  and  Gypsum  on  Cali- 
fornia Soils. 
113.   Correspondence  Courses  in  Agriculture. 
117.  The    Selection    and    Cost    of    a    Small 

Pumping  Plant. 
127.  House  Fumigation. 
129.   The  Control  of  Citrus  Insects. 
136.  MeliloUts    indica    as    a    Green-Manure 

Crop  for  California. 
144.    Oidium  or  Powdery  Mildew  of  the  Vine. 

151.  Feeding  and  Management  of  Hogs. 

152.  Some  Observations  on  the  Bulk  Hand- 

ling of  Grain  in  California. 
154.   Irrigation  Practice   in   Growing  Small 
Fruit  in  California. 


CIRCULARS 
No. 


Bovine  Tuberculosis. 

Control  of  the  Pear  Scab. 

Lettuce  Growing  in  California. 
161.   Potatoes  in  California. 
164.    Small  Fruit  Culture  in  California. 

Fundamentals   of   Sugar   Beet   Culture 
under  California  Conditions. 

The  County  Farm  Bureau. 

Feeding  Stuffs  of  Minor  Importance. 

Fertilizing  California  Soils  for  the  1918 
Crop. 

Wheat  Culture. 

The    Construction    of    the   Wood-Hoop 
Silo. 
174.   Farm  Drainage  Methods. 

178.  The  Packing  of  Apples  in  California. 

179.  Factors    of    Importance    in    Producing 

Milk  of  Low  Bacterial  Count. 


155 
157 
160 


165. 

166. 
167. 
170. 

172. 
173. 


CIRCULARS — (Continued) 


No. 

184. 

190. 

193. 

198. 

199. 

202. 

203. 
205. 
208. 

209. 
210. 
212. 
214. 

215. 
217. 

219. 
220. 
228. 
230. 

231. 

232. 

233. 
234. 

235. 

236. 


237. 


238. 
239. 


240. 
241. 


242. 
243. 


244. 


A  Flock  of  Sheep  on  the  Farm. 

Agriculture  Clubs  in  California. 

A  Study  of  Farm  Labor  in  California. 

Syrup  from  Sweet  Sorghum. 

Onion  Growing  in  California. 

County   Organizations   for   Rural    Fire 

Control. 
Peat  as  a  Manure  Substitute. 
Blackleg. 
Summary  of  the  Annual  Reports  of  the 

Farm  Advisors  of  California. 
The  Function  of  the  Farm  Bureau. 
Suggestions  "to  the  Settler  in  California. 
Salvaging  Rain-Damaged  Prunes. 
Seed  Treatment  for  the  Prevention  of 

Cereal  Smuts. 
Feeding  Dairy  Cows  in  California. 
Methods   for   Marketing  Vegetables   in 

California. 
The  Present  Status  of  Alkali. 
Unferiucnted  Fruit  Juices. 
Vineyard  Irrigation  in  Arid  Climates. 
Testing   Milk,    Cream,    and   Skim  Milk 

for  Butterfat. 
The  Home  Vineyard. 
Harvesting    and    Handling    California 

Cherries  for  Eastern  Shipment. 
Artificial  Incubation. 
Winter  Injury  to  Young  Walnut  Trees 

during  1921-22. 
Soil  Analysis  and  Soil  and  Plant  Inter- 
relations. 

The  Common  Hawks  and  Owls  of  Cali- 
fornia   from    the    Standpoint   of    the 

Rancher. 
Directions  for  the  Tanning  and  Dress- 

of  Furs. 
The  Apricot  in  California. 
Harvesting  and  Handling  Apricots  and 

Plums  for  Eastern  Shipment. 
Harvesting    and    Handling    Pears    for 

Eastern  Shipment. 
Harvesting  and  Handling  Peaches  for 

Eastern   Shipment. 
Poultry   Feeding. 
Marmalade  Juice  and  Jelly  Juice  from 

Citrus  Fruits. 
Central  Wire  Bracing  for  Fruit  Trees. 


No. 
245. 
247. 
248. 

249. 
250. 

251. 


252. 
253. 
254. 

255. 

256. 
257. 
258. 
259. 
260. 

261. 

262. 
263. 
264. 

265. 
266. 

267. 

268. 

269. 
270. 
271. 
272. 

273. 
275. 

276. 

277. 

278. 


Vine  Pruning  Systems. 

Colonization  and  Rural  Development. 

Some  Common  Errors  in  Vine  Pruning 
and  Their  Remedies. 

Replacing  Missing  Vines. 

Measurement  of  Irrigation  Water  on 
the  Farm. 

Recommendations  Concerning  the  Com- 
mon Diseases  and  Parasites  of 
Poultry  in  California. 

Supports  for  Vines. 

Vineyard  Plans. 

The  Use  of  Artificial  Light  to  Increase 
Winter  Egg  Production. 

Leguminous  Plants  as  Organic  Fertil- 
izer in   California  Agriculture. 

The  Control  of  Wild  Morning  Glory. 

The  Small-Seeded  Horse  Bean. 

Thinning  Deciduous  Fruits. 

Pear  By-products. 

A  Selected  List  of  References  Relating 
to   Irrigation  in  California. 

Sewing  Grain   Sacks. 

Cabbage  Growing  in  California. 

Tomato  Production  in  California. 

Preliminary  Essentials  to  Bovine  Tuber- 
culosis Control. 

Plant  Disease  and  Pest  Control. 

Analyzing  the  Citrus  Orchard  by  Means 
of  Simple  Tree  Records. 

The  Tendency  of  Tractors  to  Rise  in 
Front;  Causes  and  Remedies. 

Inexpensive  Lavor-saving  Poultry  Ap- 
pliances. 

An  Orchard  Brush  Burner. 

A  Farm  Septic  Tank. 

Brooding  Chicks  Artificially. 

California  Farm  Tenancy  and  Methods 
of  Leasing. 

Saving  the  Gophered  Citrus  Tree. 

Marketable  California  Decorative 
Greens. 

Home  Canning. 

Head,  Cane,  and  Cordon  Pruning  of 
Vines. 

Olive  Pickling  in  Mediterranean  Coun- 
tries. 


20m-10,'24 


